A Role for EF-Hand Proteins?
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Jan Michiels, Chuanwu Xi, Jan Verhaert and Jos Vanderleyden In bacteria, Ca2+ is implicated in a wide variety of cellular processes, including channels [1]. Calmodulin is composed of four EF- the cell cycle and cell division. Dedicated influx and efflux systems tightly hands organized in two pairs linked by a flexible control the low cytoplasmic Ca2+ levels in prokaryotes. Additionally, the central tether (Fig. 1a). The EF-hand motif was first growing number of proteins containing various Ca2+-binding motifs supports discovered in the crystal structure of parvalbumin [2] the importance of Ca2+, which controls various protein functions by affecting and comprises two nearly perpendicular α-helices protein stability, enzymatic activity or signal transduction. The existence of (named after helices E and F in parvalbumin) calmodulin-like proteins (containing EF-hand motifs) in bacteria is a long- separated by a 12-residue loop [3]. In general, standing hypothesis. Analysis of the prokaryotic protein sequences available in EF-hand Ca2+-binding proteins are recognized by the databases has revealed the presence of several calmodulin-like proteins homology in their Ca2+-binding loops. Residues 1, 3, 5, containing two or more authentic EF-hand motifs, suggesting that 7, 9 and 12 of the loop provide the ligands for calmodulin-like proteins could be involved in Ca2+ regulation in bacteria. complexing Ca2+ ions and form the basis of a pentagonal bipyramidal coordination geometry EF-hand proteins are ubiquitous in eukaryotes and (Fig. 2). Residues 1, 3 and 5 provide monodendate fulfil important regulatory or buffering roles. ligands, and residue 12 a bidendate ligand, via side- Calmodulin, a prototypical EF-hand protein, is a chain carboxylates. Residue 7 coordinates Ca2+ via a small (15–22 kDa), acidic (pI 3.9–4.3) Ca2+-binding main-chain oxygen and residue 9 coordinates Ca2+ protein that can interact with >25 distinct target indirectly via a water molecule. The identity of the proteins, thereby regulating the activity of many vital amino acids in the loop has a profound effect on the enzymes, including kinases, phosphatases, nitric Ca2+-binding characteristics of the protein [4]. Pairs oxide synthases, phosphodiesterases and ion rather than individual EF-hands constitute the http://tim.trends.com 0966-842X/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0966-842X(01)02284-3 88 Review TRENDS in Microbiology Vol.10 No.2 February 2002 (a) (b) ---Loop----- -Helix E-- --Helix F-- X Y Z-Y-X -Z 1 3 5 7 9 12 EnXXnnXXnDXDIDDGLDXXDLXXnnXXn NLENPIE EI SVNQ VN V FSG MQ M YTH CS F (c) (d) WGR T Y K A W G C TRENDS in Microbiology Fig. 2. Consensus sequence of the canonical EF-hand. The positions of the E and F helices as well as the intervening loop are given above the consensus amino acid sequence. The PROSITE EF-hand signature Fig. 1. Crystal structure of EF-hand Ca2+-binding proteins. corresponds to the 12 amino acids of the loop and the fourth amino acid (a) Ca2+–calmodulin complex. The carboxy-terminal (top) and amino- of helix F. The side chains of the coordinating residues marked X, Y, Z, terminal (bottom) lobes are linked by the central helix. The central helix −X and –Z provide oxygen atoms for the coordination of Ca2+. The and the helices flanking the Ca2+-binding loops are in pink, and the residue at position –Y bonds with a carbonyl oxygen; the residue at –Z β -sheets in yellow. (b) Type I dockerin domain from the Clostridium provides two oxygens from its carboxylate group. Ca2+ is coordinated in thermocellum cellulosome. (c) Structure of the periplasmic glucose a pentagonal bipyramid. Amino acids shown in red are not tolerated at and galactose receptor from Salmonella typhimurium (amino that position. X represents any amino acid, n is a hydrophobic amino acids 126–209). Glu205, occupying a position equivalent to that of the acid often found in the inside of the helices. residue at coordinating position 12 of the classical EF-hand, is shown in purple. (d) Structure of the Escherichia coli lytic transglycosylase Slt35 (amino acids 201–264). The Ca2+-ions are represented by blue spheres. in calmodulin, calerythrin has a high The ribbon drawing was constructed using the RasMol V2.7.1.1 phenylalanine:tyrosine ratio (13:1). A group of programme based on the coordinates in the Brookhaven Protein Data invertebrate sarcoplasmic proteins and aequorins Bank (PDB) entry (a) 1CLL, (b) 1DAQ, (c) 1GCA and (d) 1QDR. were shown to have the highest similarity with calerythrin, indicating that this protein might functional units of EF-hand proteins. Pairing of function as a Ca2+ buffer or transporter rather than neighbouring EF-hands – yielding a globular having a regulatory role [10–12]. structure – is proposed to be responsible for the More recently, we identified the EF-hand protein cooperativity of Ca2+ binding and to increase the calsymin in the Gram-negative bacterium Rhizobium affinity of the protein for this ion [5]. The EF-hands etli [13]. Calsymin has a molecular mass of 30 kDa, is in each lobe of calmodulin are linked by a short highly acidic (pI 3.6) and contains six EF-hand motifs antiparallel β-sheet (residues 7–9) and form a organized in pairs in three structurally similar compact structure. The majority of EF-hand proteins domains, A, B and C (Fig. 3; Table 1). Calsymin has contain between two and eight helix-loop-helix been shown to bind Ca2+, and plays a key role during structures that are organized in paired domains to the complex lifestyle of R. etli. R. etli induces the facilitate contact between them. formation of nodules on the roots of Phaseolus vulgaris; within these structures the bacteria are EF-hand proteins in bacteria present inside plant cells, surrounded by a plant- Calerythrin and calsymin derived membrane.