ISSN: 2456-7264 6th Issue: 28April, 2018 NO PROTEIN FOLDING NO LIFE (Fundamental principles in cell are set by protein folding) Suvendra Kumar Ray Department of Molecular Biology and Biotechnology, Tezpur University, Tezpur–784028, Assam, India, email: [email protected]. +913712275406 Prologue mechanism of folding of nrPs inside the cell. I had an opportunity to listen to a Do nrPs fold like ribosomal proteins (rPs)? I talk delivered by the Padma Bhusan P. realized that there is no clear understanding Balram, IISc Bangalore, India on one of the by the scientists regarding the folding of foundation day lectures of Institute of nrPs. Why do cells have nrPs? More I Advanced Studies in Science and started thinking about the folding of nrPs, Technology (IASST), Guwahati, India. The several exciting ideas occurred to me. I talk had covered proteinaceous toxins (prTs) shared the ideas with my students and produced by animals. Interestingly many colleagues. Ultimately I got an impression prTs are synthesized by enzymes directly that protein folding is the main fundamental using amino acids as substrates without problem in cell which governs several translation (tRNA, ribosome and mRNA). fundamental principles in cell. So these peptides or proteins belong to the The role of the central dogma and non-ribosomal proteins (nrPs) category. ribosome in protein folding Though previously I was aware of some If proteins can be synthesized nrPs, it is after hearing the above without using ribosome and mRNA as in deliberation that I got more curious about case of nrPs, then why not all proteins inside nrPs. During the question answer sessions, I the cell got synthesized as nrPs during had the opportunity to ask a question on the evolution, which would have obviously avoided the requirement of DNA and RNA then the enzyme E3 will add A4 to A1A2A3 in life, and proteins would have also been to make A1A2A3A4, then enzyme E4 will the genetic material. So where was the add A5 to A1A2A3A4 to make difficulty? I started analyzing enzyme A1A2A3A4A5, and finally the enzyme E5 mediated synthesis of nrPs in cell. A will add A6 to A1A2A3A4A5 to schematic representation of a hypothetical A1A2A3A4A5A6. It is simple for E1 to synthesis of a hexapeptide as nrPs is recognize A1 and A2 to synthesize A1A2. described in Fig. 1. This helped us to E2 will bind to E1E2 and will join with E3. understand the problem associated with the E1E2 will attain different conformations. To nrPs synthesis. It is obvious to realize that which conformation it will bind. The the folding of these proteins is indeed a enzyme will recognize the amino acid problem. sequence or the structure of the peptide. How the enzyme E5 will recognize the pentapeptide A1A2A3A4A5? If it recognizes the peptide by recognizing each amino acid, then the entire peptide is to remain in unfolded state, if it recognizes the structure, then it should have a structure which other peptides should not attain. As Fig 1. A schematic presentation of the peptide size increases, it is difficult for enzymatic synthesis of a hexapeptide (nrPs) an enzyme to either recognize the longer Enzymatic synthesis of a hexapeptide is sequence of amino acids or to recognize the schematically shown. E1 to E6 are enzymes conformation of the peptide to add the presented as semi-circles. A1 to A6 are incoming amino acids. If it recognizes amino acids. conformation, then it will not be specific for the peptide because different peptides might Let’s consider an example of the synthesis have the same conformation. If it recognizes of a hexa-peptide peptide the primary sequence, then the (A1A2A3A4A5A6) without involving RNA protein/peptide has to remain in fully template. In the first step, enzyme E1 will unfolded state till it gets completely add A1 and A2 to make A1A2, then enzyme synthesized. Once a protein remains E2 will add A3 to A1A2 to make A1A2A3, completely unfolded till it gets completely synthesized, then how to know that the of ribosome and the ribosomal RNA in synthesis of the protein has reached its final protein folding (Bera et al., 1994; Gupta, step? Either termination amino acid is 1999; Das et al., 1996; Chattopadhyay et al., required in protein synthesis or the protein is 1996). I shall regard Prof. Dasgupta’s to be circular in nature. Considering these contributions to this field as equivalent to points, it seems that folding of a protein, the contribution of E. Chargaff’s whose synthesis is not guided by a template, biochemistry research towards the DNA is not simple. The few nrPs are assumed to double helix discovery. It is now beyond fold spontaneously and these can be doubt that protein folding is a co- considered as exceptions to the rule. translational process (Jacobs and Shakhnovich, 2017) and the nascent Is protein folding so fundamental in polypeptides fold in the exit channel of cellular life that it resulted in the evolution ribosome (Sohmen et al., 2015; Ito, 2016). of template dependent life in cell, i.e. DNARNAProtein, the central dogma in Ribosome size is one of the main molecular biology? Ribosome, the site of differences between prokaryotes and protein synthesis, is the most complex eukaryotes: prokaryotes have 70S ribosomes structure present inside a cell. If peptide while eukaryotes have 80S ribosomes. This bonds can be formed in nrPs without structural difference has been exploited to ribosome, then why did the evolution of use antibiotics to get rid of bacterial such a complex structure like ribosome infections in humans. Why ribosome occur? Is the main function of ribosome structure is so different between the two inside the cell to fold proteins during their types of cell? It indicates that ribosome has synthesis? It is exciting that many research to execute some different functions between articles published in leading journals have these cells. We know that ribosome in recently described the role of ribosome in eukaryotes remain bound to the membrane protein folding (Das et al, 2008; Nilsson et of endoplasmic reticulum (ER) to carry out al., 2015; Holtkamp et al., 2015; Javed et co-translational translocation of proteins al., 2017). While searching literature, it was across the ER membrane. Formation of very exciting to find that almost two decades disulfide bond in proteins in eukaryotes ago, Prof. Chanchal Dasgupta from Kolkata occurs inside the ER lumen. Proteins that are University, India carried out several to be secreted out to the exterior, Golgi pioneering experiments to describe the role complex, and the cell membrane - all pass through the ER. The co-translational synonymous changes in coding sequences translocation of proteins is not found in (Kudla et al., 2009; Hu et al., 2013; Zhou et prokaryotes. The co-translational al., 2013). Now it is realized that the translocation of proteins through ER is a sequence of codons in mRNA not only fundamental requirement for different contains the information about amino acid protein modifications and folding of sequence for a protein, it also contains membrane and secreted proteins. This may information for protein structure (Ray et al., be a fundamental cause for the spatial and 2014; Mitra et al., 2016). temporal separation of transcription and Protein synthesis by ribosome is translation events in eukaryotes and the fundamentally different from the synthesis reason for evolution of nucleus. I believe ER of DNA and RNA in cell: unlike the to be the first membrane bound organelle synthesis of DNA and RNA where the evolved before the eukaryotic cell became nucleotides diffuse directly to the enzyme the way it is and so the evolution of 80S active site, amino acids are brought to the ribosome in eukaryotes. ribosome active site by tRNA for protein Co-translational protein folding and synthesis. Amino acid incorporation for codon degeneracy in amino acids protein synthesis involves several steps as Anfinsen suggested that the three follows: (i). amino acid tagged tRNAs dimensional structure of a protein is (charged tRNAs) are brought by the dependent upon its primary structure, i.e. elongation factors to the ribosome; (ii). the amino acid sequence (Anfinsen, 1972; correct codon-anticodon pairing is detected 1973). This theory got conclusively by the smaller subunit of the ribosome challenged recently in 2007, when it was (30S/40S) which triggers a movement of the demonstrated that a protein with unchanged tRNA acceptor stem bringing the amino acid amino acid sequence attained a different to the proximity of the peptidyl tRNA in the conformation due to a synonymous change bigger ribosomal subunit, and (iii). the in the nucleotide sequence in the coding peptidyl transferase activity in the larger region (Kimchi-Sarfaty et al., 2007). After subunit (50S/60S) catalyzes the formation of this seminal discovery, many more a peptide bond with the incoming amino evidences were demonstrated suggesting the acid and simultaneously the transfer of the importance of co-translational protein peptide from one tRNA to the other tRNA folding and protein structure by doing occurs (Ogle et al., 2003). Therefore, translation is the most complex phenomenon protein folding and structure is an important inside the cell. This complex pathway has factor for the assignment of degeneracy to evolved to make the protein synthesis a amino acids. A hypothetical description on slower (Choi and Puglisi, 2017) and the significance of amino acids in protein accurate process than DNA and RNA structure and codon degeneracy for the synthesis, which may be also to assist amino acid is given in Fig. 2. folding proteins correctly during their synthesis (Pechmann et al., 2013). In the genetic code table, 18 out of the 20 amino acids are encoded by more than one codon, known as codon degeneracy (Satapathy et al., 2016).