TRANSLATION

Course Content - i. Mechanism of Protein Synthesis ii. Protein Sorting iii. Inhibitors of Protein Synthesis iv. Post -translational Modifications v. HSPs

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THREE BASIC STAGES OF PROTEIN SYNTHESIS

I. Initiation II. Elongation III. Termination

Similar in bacteria & eukaryotes.

Various Proteins & Factors Involved in Translation

Gurumayum Suraj Sharma INITIATION OF TRANSLATION ‹ INITIATION ‹ Encompasses all steps preceding formation of the peptide bond between the first two amino acids in the polypeptide chain. ‹ Initiation involves an mRNA MOLECULE , a , a SPECIFIC INITIATOR tRNA , PROTEIN INITIATION FACTORS (IF) & GTP (GUANOSINE TRIPHOSPHATE).

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INITIATION - BACTERIA

First step in initiation - ‹ Interaction of 30S [small] ribosomal subunit to which IF -1 & IF -3 are bound with region of mRNA containing AUG initiation codon . ‹ IF -3 aids in binding of subunit to mRNA & prevents binding of 50S ribosomal subunit to 30S subunit.

‹ AUG initiation codon alone not sufficient to indicate where 30S subunit should bind to mRNA ‹ A sequence upstream [to 5’ side in leader of mRNA] of AUG called Ribosome-binding site [RBS] also needed.

Gurumayum Suraj Sharma ‹ John Shine & Lynn Dalgarno [1970s] hypothesized that purine -rich RBS sequence (5’-AGGAG -3’ or some similar sequence) & sometimes other nucleotides in this region could pair with a complementary pyrimidine- rich region [5’-UCCUCC -3’] at 3’ end of 16S rRNA .

Shine-Dalgarno Sequence

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‹ Joan Steitz ‹First to demonstrate pairing experimentally. ‹ The mRNA RBS region is now commonly known as the Shine - Dalgarno sequence.

Sequences Involved in Binding of to mRNA in Initiation of Protein Synthesis in Prokaryotes.

Gurumayum Suraj Sharma ‹ Most RBSs are 8-12 nucleotides upstream from initiation codon. ‹ MODEL - Formation of complementary bp between mRNA & 16S rRNA allows small ribosomal subunit to locate true sequence in mRNA for initiation. ‹ GENETIC EVIDENCE ‹ If Shine-Dalgarno sequence mutated - Its possible pairing with 16S rRNA sequence significantly diminished or prevented, mutated mRNA cannot be translated. ‹ Likewise, if rRNA sequence complementary to Shine–Dalgarno sequence is mutated , mRNA translation cannot occur. ‹ Loss of translatability - A result of mutations in one or other RNA partner could be caused by effects unrelated to the loss of pairing of the two RNA segments, a more elegant experiment was done. ‹ Mutations were made in the Shine-Dalgarno sequence to abolish pairing with wild-type rRNA sequence, and compensating mutations were made in the rRNA sequence so that the two mutated sequences could pair. ‹ In this case, mRNA translation occurred normally, indicating the importance of the pairing of the two RNA segments. Gurumayum Suraj Sharma

Next step in initiation ‹ Binding of a special initiator tRNA to AUG start codon to which 30S subunit is bound. ‹ In both prokaryotes and eukaryotes ‹ AUG initiator codon specifies methionine. ‹ Thus, newly made proteins in both types of organisms begin with methionine. ‹ Methionine may be removed later in many cases.

Gurumayum Suraj Sharma ‹ BACTERIA - Initiator tRNA is tRNA.fMet [with anticodon 5’-CAU -3’] bind to AUG start codon. ‹ This tRNA carries a modified form of methionine, formylmethionine (fMet) ‹ Formyl group added to amino group of methionine. ‹ First, methionyl -tRNA synthetase catalyzes addition of methionine to tRNA. ‹ Then enzyme TRANSFORMYLASE adds formyl group to methionine. ‹ The resulting molecule is designated fMet -tRNA

‹ When AUG codon in mRNA molecule is encountered at a position other than start of amino acid-coding sequence ‹ Different tRNA [called tRNA.Met], is used to insert methionine at that point in polypeptide chain.

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‹ Initiator tRNA , fMet - tRNA.fMet , is brought to 30S subunit-mRNA complex by IF - 2, which also carries a molecule of GTP. ‹ Initiator tRNA binds to subunit in P site. ‹ All aminoacyl-tRNAs that come to ribosome bind to A site . ‹ However, IF-1 bound to 30S subunit is blocking A site so that only P site is available for initiator tRNA to bind to. ‹ Thus, 30S initiation complex formed ‹ Consists of mRNA, 30S subunit, initiator tRNA, and the initiation factors. ‹ Next, 50S ribosomal subunit binds, leading to GTP hydrolysis and release of three initiation factors. ‹ The final complex is called the 70S initiation complex. Gurumayum Suraj Sharma INITIATION IN EUKARYOTES ‹ Similar in eukaryotes, however, more complex & involves many more initiation factors, called eukaryotic initiation factors [eIF] MAIN DIFFERENCES ‹ Initiator methionine is unmodified , although a special initiator tRNA still brings it to the ribosome ‹ Shine-Dalgarno sequences are not found in eukaryotic mRNAs. ‹ Instead, eukaryotic ribosome uses another way to find AUG initiation codon. ‹ First, a eukaryotic initiator factor eIF-4F- A multimer of several proteins, including eIF-4E, the capbinding protein [CBP] - ‹ Binds to cap at 5’ end of mRNA. ‹ Then, a complex of 40S ribosomal subunit with initiator Met- tRNA, several eIF proteins, & GTP binds, together with other eIFs, & moves along mRNA, scanning for initiator AUG codon. ‹ AUG codon is embedded in a short sequence- called Kozak sequence [after Marilyn Kozak ], indicates that it is initiator codon. - Process is called scanning model for initiation. Gurumayum Suraj Sharma

‹ AUG codon is almost always the first AUG codon from 5’ end of mRNA ‹ But, to be an initiator codon, it must be in an appropriate sequence context. ‹ Once 40S subunit finds AUG, it binds to it, and then 60S ribosomal subunit binds, displacing eIFs (except for eIF-4F, which is needed for the subsequent initiation of translation ), producing 80S initiation complex with initiator Met-tRNA bound to mRNA in P site of ribosome. ‹ Poly(A) tail of eukaryotic mRNA also plays a role in translation . ‹ Poly(A) Binding Protein II [PABPII] bound to poly(A) tail also binds eIF-4G [one of the proteins of eIF-4F at cap] , thereby looping the 3’ end of mRNA close to 5’ end. ‹ Thus, poly(A) tail stimulates initiation of translation .

Gurumayum Suraj Sharma Initiation of protein synthesis: ‹ A 30S ribosomal subunit, mRNA, initiator fMet-tRNA, and initiation factors form a 30S initiation complex. ‹ Next, the 50S ribosomal subunit binds, forming a 70S initiation complex. ‹ During this event, the initiation factors are released and GTP is hydrolyzed.

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ELONGATION OF POLYPEPTIDE CHAIN

‹ After initiation is complete, the next stage is elongation. ‹ The addition of amino acids to the growing polypeptide chain one by one- as they take place in bacteria. ‹ THREE STEPS : 1. Aminoacyl -tRNA (charged tRNA) binds to ribosome in A site. 2. A peptide bond forms. 3. Ribosome moves (Translocates) along mRNA one codon.

‹ As with initiation, elongation requires accessory protein factors, called elongation factors [EF] , and GTP. ‹ Elongation is similar in eukaryotes

Gurumayum Suraj Sharma Binding of Aminoacyl -tRNA ‹ At start of elongation, anticodon of fMet -tRNA is H-bonded to AUG initiation codon in P site of the ribosome. ‹ Next codon in mRNA is in A site ‹ Next, the appropriate aminoacyl-tRNA binds to codon in A site. ‹This aminoacyl-tRNA is brought to ribosome bound to EF -Tu -GTP [a complex of the protein elongation elongation factor, EF- G].

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‹ An EF-G-GTP complex binds to ribosome, GTP is hydrolyzed, and translocation of ribosome occurs along with displacement of uncharged tRNA away from P site. ‹ It is possible that GTP hydrolysis changes the structure of EF-G, which facilitates translocation event.

Gurumayum Suraj Sharma ‹ Translocation similar in eukaryotes - ‹ Elongation factor in this case is eEF-2, which functions like bacterial EF-G. ‹ Uncharged tRNA moves from P site and then binds transiently to E site in 50S ribosomal subunit, blocking next aminoacyl-tRNA from binding to A site until translocation is complete & peptidyl-tRNA is bound correctly in P site. ‹ Once that has occurred, uncharged tRNA is then released from ribosome. ‹ After translocation, EF-G is released and then reused.

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‹ During translocation , peptidyl-tRNA remains attached to its codon on mRNA ‹ Since ribosome has moved, peptidyl-tRNA is now located in the P site (hence name peptidyl site ). ‹ After completion of translocation, A site is vacant. ‹ An aminoacyl–tRNA with correct anticodon binds to newly exposed codon in A site, reiterating the process already described. ‹ The whole process is repeated until translation terminates at a stop codon.

Gurumayum Suraj Sharma ‹ Once ribosome moves away from initiation site on mRNA, another initiation event occurs [occurs both in Prokaryotes & Eukaryotes]. ‹ The process is repeated until, typically, several ribosomes are translating each mRNA simultaneously. ‹ The complex between an mRNA molecule and all the ribosomes that are translating it simultaneously is called a polyribosome, or polysome. ‹ Each ribosome in a polysome translates the entire mRNA and produces a single, complete polypeptide. ‹ Polyribosomes enable a large number of polypeptides to be produced quickly and efficiently from a single mRNA.

Polysome- A number of ribosomes, each translating same mRNA sequentially

TERMINATION OF TRANSLATION ‹ Termination - Signalled by one of three stop codons (UAG, UAA, and UGA), which are the same in prokaryotes and eukaryotes. ‹ Stop codons do not code for any amino acid, so no tRNAs in cell have anticodons for them. ‹ Ribosome recognizes a stop codon with the help of proteins called RELEASE FACTORS [RF] , which have shapes mimicking that of a tRNA including regions that read the codons ‹ Initiate a series of specific termination events.

‹ In E. coli , there are three RFs , two of which read stop codons: ‹ RF1 recognizes UAA and UAG ‹ RF2 recognizes UAA and UGA. ‹ Binding of RF1/RF2 to a stop codon triggers peptidyl transferase to cleave polypeptide from tRNA in P site. ‹ The polypeptide then leaves the ribosome .

Gurumayum Suraj Sharma ‹ Next, RF3-GDP binds to ribosome, stimulating release of RF from stop codon and ribosome. ‹ GTP now replaces GDP on RF3, and RF3 hydrolyses GTP, which allows RF3 to be released from ribosome. ‹ An additional important step is the deconstruction of remaining complex of ribosomal subunits, mRNA, and uncharged tRNA so that the ribosome and tRNA may be recycled. ‹ In E. coli , Ribosome Recycling Factor [RRF] [Shape of which mimics that of a tRNA] Binds to A site. ‹ Then EF-G binds, causing translocation of the ribosome and thereby moving RRF to the P site and the uncharged tRNA to the E site. ‹ The RRF releases uncharged tRNA, and EF-G releases RRF, causing the two ribosomal subunits to dissociate from the mRNA.

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‹ In eukaryotes, termination process is similar to bacteria. ‹ In this case, a single release factor [Eukaryotic release factor 1 (eRF1)] recognizes all three stop codons ‹ eRF3 stimulates termination events. ‹ Ribosome recycling occurs in eukaryotes, but there is no equivalent of RRF. ‹ A polypeptide folds during translation process.

Gurumayum Suraj Sharma Termination of translation ‹ Ribosome recognizes a chain termination codon (UAG) with aid of release factors . ‹ A release factor reads the stop codon, initiating a series of specific termination events leading to the release of the completed polypeptide. ‹ Subsequently, ribosomal subunits, mRNA, and uncharged tRNA separate. ‹ In bacteria, this event is stimulated by ribosome recycling factor (RRF) and EF- G.

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PROTEIN SORTING IN CELL

‹ In Bacteria & Eukaryotes , some proteins may be secreted ‹ In EUKARYOTES , some other proteins must be placed in different cell compartments [such as nucleus, mitochondrion, chloroplast, & lysosome ]. ‹ Sorting of proteins to their appropriate compartments is under genetic control, in that specific “signal” or “leader” sequences on proteins direct them to correct organelles. ‹ Similarly, in bacteria, certain proteins become localized in membrane. ‹ Others are secreted.

Gurumayum Suraj Sharma SYNTHESIS OF PROTEINS ON MEMBRANE - BOUND VERSUS FREE RIBOSOMES

Polypeptides are synthesized at two distinct locales within the cell 1. Certain polypeptides are synthesized on ribosomes attached to RER. These include: o Secreted proteins o Integral membrane proteins o Soluble proteins ° That reside within compartments of the endomembrane system [ER, Golgi complex, Lysosomes, Vesicles, Plant Vacuoles, etc.]

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2. Other polypeptides are synthesized on “free” ribosomes, i.e., on ribosomes not attached to RER, & are subsequently released into cytosol. This class includes o Proteins destined to remain in the cytosol (such as the enzymes of glycolysis and the proteins of the cytoskeleton) o Peripheral proteins of cytosolic surface of membranes (Spectrins & Ankyrins that are weakly associated with plasma membrane’s cytosolic surface) o Proteins that are transported to nucleus o Proteins to be incorporated into peroxisomes, chloroplasts & mitochondria . ° Proteins in the latter two groups are synthesized to completion in cytosol & imported post-translationally into the appropriate organelle across its boundary membranes

Gurumayum Suraj Sharma How proteins are secreted from eukaryotic cell…..?

‹ Such proteins passage through ER & Golgi apparatus . ‹ Gunther Blobel, B. Dobberstein, and colleagues [1975] ‹ Secreted & other proteins sorted by Golgi initially contain extra amino acids at amino terminal end . ‹ Led to SIGNAL HYPOTHESIS ‹ Proteins sorted by Golgi bind to ER by a hydrophobic amino terminal extension (signal sequence) to the membrane that is subsequently removed and degraded. ‹ Blobel - Nobel Prize in Physiology or Medicine [1999]

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SIGNAL HYPOTHESIS ‹ Site of synthesis of a protein is determined by sequence of amino acids in N-terminal portion of the polypeptide, which is the first part to emerge from the ribosome during protein synthesis. 1. Secretory proteins contain a signal sequence at their N- terminus that directs emerging polypeptide & ribosome to ER membrane. 2. The polypeptide moves into cisternal space of ER through a protein-lined, aqueous channel in ER membrane. o The polypeptide moves through membrane as it is being synthesized, i.e., co-translationally

Gurumayum Suraj Sharma 3. Protein transport across ER membrane can also occur post - translationally (i.e., after synthesis) . o In this process, the polypeptide is synthesized totally in cytosol and then imported into ER lumen. o The post-translational pathway is utilized more heavily in yeast than in mammals for import into ER.

Synthesis of a secretory protein on a membrane-bound ribosome of RER.

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SYNTHESIS OF INTEGRAL MEMBRANE PROTEINS ON MEMBRANE -BOUND RIBOSOMES

‹ Integral membrane proteins also synthesized on membrane- bound ribosomes of the ER. ‹ These proteins are translocated into ER membrane as they are synthesized (i.e., co -translationally ) ‹ Unlike soluble secretory & lysosomal proteins [which pass entirely through ER membrane during translocation], integral proteins contain hydrophobic trans -membrane segments ‹ Thus, shunted directly from channel of TRANSLOCON into lipid bilayer. Translocon- Translocation Channel

Gurumayum Suraj Sharma ‹ As a polypeptide passes through translocon , this lateral “GATE ” in channel continually opens & closes ‹ Gives each segment of polypeptide an opportunity to partition itself according to its solubility - ‹ Into either aqueous compartment within translocon channel or surrounding hydrophobic core of lipid bilayer. ‹ Those segments of nascent polypeptide that are sufficiently hydrophobic will “dissolve ” into lipid bilayer & ultimately become trans-membrane segments of an integral membrane protein.

Synthesis of an integral membrane protein

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INHIBITORS OF PROTEIN SYNTHESIS Protein Synthesis Is Inhibited by Many Antibiotics and Toxins ‹ Protein synthesis - Central function in cellular physiology ‹ Thus, the primary target of antibiotics & toxins . ‹ These antibiotics inhibit protein synthesis [mainly] in bacteria. ‹ The subtle differences in bacterial & eukaryotic protein synthesis, are sufficient that most of the compounds are relatively harmless to eukaryotic cells. ‹ Natural selection has favoured evolution of compounds that exploit minor differences in order to affect bacterial systems selectively, such that these biochemical weapons are synthesized by some microorganisms & are extremely toxic to others. ‹ Nearly every step in protein synthesis can be specifically inhibited by one antibiotic or another ‹ Thus, antibiotics have become valuable tools in study of protein biosynthesis.

Gurumayum Suraj Sharma Puromycin [from Streptomyces alboniger ] ‹ One of the best-understood inhibitory antibiotics. ‹ Structure very similar to 3’ end of an aminoacyl-tRNA . ‹ Thus enabling it to bind to ribosomal A site & participate in peptide bond formation, producing peptidyl-puromycin . ‹ However, since puromycin resembles only 3’ end of tRNA, it does not engage in translocation & dissociates from ribosome shortly after it is linked to carboxyl terminus of peptide. ‹ Thus prematurely terminates polypeptide synthesis.

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Tetracyclines ‹ Inhibit protein synthesis in bacteria by blocking A site on ribosome, preventing binding of aminoacyl-tRNAs.

Chloramphenicol ‹ Inhibits protein synthesis by bacterial (& mitochondrial & chloroplast) ribosomes by blocking peptidyl transfer. ‹ It does not affect cytosolic protein synthesis in eukaryotes.

Cycloheximide ‹ Blocks peptidyl transferase of 80S eukaryotic ribosomes but not that of 70S bacterial (and mitochondrial and chloroplast) ribosomes.

Streptomycin [A basic trisaccharide ] ‹ Causes misreading of genetic code (in bacteria) at relatively low concentrations ‹ Inhibits initiation at higher concentrations. Gurumayum Suraj Sharma Other inhibitors of protein synthesis ‹ Notable because of their toxicity to humans and other mammals. ‹ Diphtheria toxin catalyzes ADP-ribosylation of a diphthamide (a modified histidine) residue of eukaryotic elongation factor eEF2, thereby inactivating it. ‹ Ricin - An extremely toxic protein of castor bean, inactivates 60S subunit of eukaryotic ribosomes by depurinating a specific adenosine in 23S rRNA.

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PROCESSING OF NEWLY SYNTHESIZED PROTEINS POSTTRANSLATIONAL MODIFICATIONS ‹ Polypeptide chains, like RNA transcripts, are modified once they have been synthesized. ‹ This additional processing broadly described as Posttranslational Modifications [PTMs]. ‹ Critical to functional capability of final protein product. ‹ PTMs increase functional diversity of proteome by covalent addition of functional groups or proteins, proteolytic cleavage of regulatory subunits, or degradation of entire proteins. ‹ Chemical modifications that play a key role in functional proteomic because they regulate activity, localization, and interaction with other cellular molecules such as proteins, nucleic acids, lipids and cofactors ‹ Influence almost all aspects of normal cell biology & pathogenesis. ‹ Thus, identifying and understanding PTMs is critical in study of cell biology and disease treatment and prevention.

Gurumayum Suraj Sharma MAJOR POSTTRANSLATIONAL MODIFICATIONS

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Increase in complexity from level of genome to proteome is further facilitated by protein PTMs ‹ PTMs are key mechanisms to increase proteomic diversity. ‹ While genome comprises 20,000 -25,000 genes , proteome is estimated to encompass over 1 million proteins . ‹ Changes at transcriptional and mRNA levels increase size of transcriptome relative to the genome ‹ The myriad of different PTMs exponentially increases complexity of proteome relative to both transcriptome and genome.

Gurumayum Suraj Sharma Examples of posttranslational modification: 1. N-terminus amino acid is usually removed or modified o Eg., In bacterial polypeptides , Formyl group [or entire formylmethionine residue] is usually removed enzymatically. o In eukaryotic , amino group of initial methionine residue is often removed, and the amino group of the N-terminal residue may be modified [acetylated ] 2. Individual amino acid residues are sometimes modified o Eg., Phosphates may be added to hydroxyl groups of certain amino acids, such as tyrosine. o Such modifications create negatively charged residues that may form an ionic bond with other molecules. o Phosphorylation - Extremely important in regulating many cellular activities o Result of action of enzymes - Kinases. o At other amino acid residues, Methyl or Acetyl groups may be added enzymatically o Can affect function of modified polypeptide chain.

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3. Carbohydrate side chains are sometimes attached. o Added covalently, producing glycoproteins , an important category of cell-surface molecules. -Glycosylation o Responsible for specifying antigens in the ABO blood-type system in humans.

4. Polypeptide chains may be trimmed. o Eg., gene is first translated into a longer molecule. o Enzymatically trimmed to final form.

Gurumayum Suraj Sharma 5. Signal sequences are removed. o At N-terminal end of some proteins is a sequence of up to 30 amino acids that plays an important role in directing the protein to the location in the cell in which it functions. o Called a Signal Sequence [determines final destination of protein in cell] o Called Protein Targeting . o Eg., proteins whose fate involves secretion or proteins that are to become part of plasma membrane are dependent on specific sequences for their initial transport into the lumen of the ER. o While signal sequence of various proteins with a common destination might differ in primary amino acid sequence, they share many chemical properties. o Eg., those destined for secretion all contain a string of up to 15 hydrophobic amino acids preceded by a positively charged amino acid at the N-terminus of the signal sequence. o Once polypeptides transported, but before they achieve final functional status, signal sequence is enzymatically removed from them. Gurumayum Suraj Sharma

6. Polypeptide chains are often complexed with metals. o Tertiary & quaternary levels of protein structure often include and are dependent on metal atoms . o Functional protein is thus a molecular complex that includes both polypeptide chains and metal atoms. o Hemoglobin , which contains four iron atoms along with its four polypeptide chains. o Also Myoglobin, Carbonic anhydrase , etc.

Gurumayum Suraj Sharma HEAT SHOCK PROTEINS [HSPs] ‹ Large class of proteins conserved throughout evolution & exist in prokaryote and eukaryote organisms. ‹ Play crucial role in PROTEIN HOMEOSTASIS [PROEOSTASIS] . ‹ Found in all major cellular compartments. ‹ When normal cells exposed to heat, cells begin to intensively synthesize stress proteins [HSPs] that have chaperone function. ‹ They are therefore called CHAPERONES. ‹ Chaperones regulate changes in protein arrangement through membranes during transport. ‹ Regulate CONFORMATION [arrangement of proteins at slight damage ]. ‹ Molecular chaperones in folding of newly synthesized proteins ‹ They participate in their transport across membranes, as well as their integration into various organelles.

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CHAPERONES ‹ Inhibit of change of proteins conformation at increased temperature to 42 °C in cell. ‹ Cell can adapt & range of this adaptation is limited ‹ Cell is damaged by intense stress. ‹ Molecular chaperones have important functions in maintaining cell homeostasis and cellular response to stress. ‹ Specifically, the following properties and functions: I. Prevent proteins aggregation in folding & unfolding of protein II. Affect production and kinetics in protein folding III. Are involved in transfer of cellular proteins between compartments IV. Have a regulatory function in signal transduction

Gurumayum Suraj Sharma STRESS PROTEINS ‹ Include molecular chaperones & proteins induced by high temperatures ‹ Also includes proteases, ubiquitin & dehydrins [plants]. ‹ The presence of HSPs was demonstrated in all living organisms. ‹ Synthesized as response to other stress factors too ‹ Cold, UV radiation, bacterial & viral infections, heavy metals, pesticides, etc. ‹ Activation of HSPs genes - Universal answer to stress ‹ Includes changes in transcription & translation. ‹ Allows bacterial, plant & animal cells to survive damage caused by external & internal environment. ‹ Preservation HSPs response in evolution of all living matter shows that it benefits majority of cell types. ‹ The answer HSPs is not permanent and ends after returning cells to normal conditions.

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HSPs are involved in all important processes associated with growth - ‹ Segmentation, DNA synthesis, transcription, translation, and protein folding & transport through membranes . ‹ Eg. HSP70 [Mediator of protein folding], HSP60 [Chaperonin, supports post-translational accumulation of polypeptides], HSP83 [Belongs to HSP90 proteins those genes code also chaperon proteins]. ‹ In stress conditions HSPs are involved in repair of damaged proteins. ‹ In relation to immune system, HSPs significantly influence of induction & processes immune responses ‹ HSPs are considered to be cytokines in terms of regulation of immune mechanisms. HSPs Classified Into Several Classes Based On Molecular Weight - ‹ Such as HSP90 (85-90 kDa), HSP70 (68-73 kDa), HSP60, HSP47 and small HSPs (12-43 kDa). ‹ HSP90 - Protein family HSP90 is important in formation of steroid receptor complexes. ‹ In particular, the stress protein HSP90 is a key chaperone enabling stabilized many proteins involved in malignant transformation of cells. ‹ Multiple increase of activity HSP 90 in tumor cells is an important condition for the activation of various signaling pathways sustaining cell proliferative potential and prevents apoptosis. Gurumayum Suraj Sharma HSP70 - ‹ Essential for protein synthesis, translocation and storage. ‹ HSP70 can be found in different cellular compartments (nuclear, cytosolic, mitochondrial, endoplasmic reticulum, etc.). ‹ There exist many of proteins that are tied to Family HSP 70. ‹ Some present only under stress conditions (highly INDUCIBLE ), while some are present in cells under normal growth conditions (CONSTITUTIVE )

HSP60 - ‹ Proteins of this family are also called chaperonins. ‹ They are oligomers of protein complexes. ‹ HSP60 in eukaryotes are synthesized in cytoplasm and transported into the mitochondria. ‹ Associated with mitochondrial matrix & participate in production and transport of proteins into mitochondria.

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‹ Importance of HSPs in creation of proteins ‹ Reflected in the fact that number of HSPs genes expressed at high levels during normal cell growth. ‹ Oxygen radicals, toxins, stress & inflammatory processes often lead to accumulation of denatured & aberrantly folded [misfolded] proteins ‹ Induce synthesis of HSPs. ‹ Synthesis of denatured proteins & cellular proteins is increased with increase in temperature or under stress. ‹ HSPs bind to newly synthesized & denatured proteins & help their folding to right native conformation. Correctly Folded Native Protein

Denature/misfolded Protein

HSP/Chaperone

Gurumayum Suraj Sharma Source Peter J. Russell. Genetics Klug & Cummings. Concept of Genetics Gerald Karp. Cell &

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