title: A 1.3-A structure of zinc-bound N-terminal domain of calmodulin elucidates potential early ion-binding step.
abstract: Calmodulin (CaM) is a 16.8-kDa calcium-binding protein involved in calcium-signal transduction. It is the canonical member of the EF-hand family of proteins, which are characterized by a helix-loop-helix calcium-binding motif. CaM is composed of N- and C-terminal globular domains (N-CaM and C-CaM), and within each domain there are two EF-hand motifs. Upon binding calcium, CaM undergoes a significant, global conformational change involving reorientation of the four helix bundles in each of its two domains. This conformational change upon ion binding is a key component of the signal transduction and regulatory roles of CaM, yet the precise nature of this transition is still unclear. Here, we present a 1.3-A structure of zinc-bound N-terminal calmodulin (N-CaM) solved by single-wavelength anomalous diffraction phasing of a selenomethionyl N-CaM. Our zinc-bound N-CaM structure differs from previously reported CaM structures and resembles calcium-free apo-calmodulin (apo-CaM), despite the zinc binding to both EF-hand motifs. Structural comparison with calcium-free apo-CaM, calcium-loaded CaM, and a cross-linked calcium-loaded CaM suggests that our zinc-bound N-CaM reveals an intermediate step in the initiation of metal ion binding at the first EF-hand motif. Our data also suggest that metal ion coordination by two key residues in the first metal-binding site represents an initial step in the conformational transition induced by metal binding. This is followed by reordering of the N-terminal region of the helix exiting from this first binding loop. This conformational switch should be incorporated into models of either stepwise conformational transition or flexible, dynamic energetic state sampling-based transition.
authors.affiliation: Biological Sciences Collegial Division, The University of Chicago, Chicago, IL 60637, USA.
authors.forename: Julia T
authors.initials: JT
authors.lastname: Warren
authors.forename: Qing
authors.initials: Q
authors.lastname: Guo
authors.forename: Wei-Jen
authors.initials: WJ
authors.lastname: Tang
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism; D001059:Apoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology; D002147:Calmodulin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D003432:Cross-Linking Reagents; D003460:Crystallization; D018360:Crystallography, X-Ray; D020832:EF Hand Motifs; D006801:Humans; D008024:Ligands; D021962:Membrane Microdomains; D008958:Models, Molecular; D011485:Protein Binding; D011487:Protein Conformation / Q000187:drug effects*; D017434:Protein Structure, Tertiary; D015032:Zinc / Q000737:chemistry*
chemical_list: D001059:Apoproteins; D002147:Calmodulin; D003432:Cross-Linking Reagents; D008024:Ligands; D000262:Adenylyl Cyclases; D015032:Zinc; D002118:Calcium