title: Structures of Anabaena calcium-binding protein CcbP: insights into Ca2+ signaling during heterocyst differentiation.
abstract: Ca2+-binding proteins play pivotal roles in both eukaryotic and prokaryotic cells. CcbP from cyanobacterium Anabaena sp. strain PCC 7120 is a major Ca2+-binding protein involved in heterocyst differentiation, a process that forms specialized nitrogen-fixing cells. The three-dimensional structures of both Ca2+-free and Ca2+-bound forms of CcbP are essential for elucidating the Ca2+-signaling mechanism. However, CcbP shares low sequence identity with proteins of known structures, and its Ca2+-binding sites remain unknown. Here, we report the solution structures of CcbP in both Ca2+-free and Ca2+-bound forms determined by nuclear magnetic resonance spectroscopy. CcbP adopts an overall new fold and contains two Ca2+-binding sites with distinct Ca2+-binding abilities. Mutation of Asp38 at the stronger Ca2+-binding site of CcbP abolished its ability to regulate heterocyst formation in vivo. Surprisingly, the β-barrel subdomain of CcbP, which does not participate in Ca2+-binding, topologically resembles the Src homology 3 (SH3) domain and might act as a protein-protein interaction module. Our results provide the structural basis of the unique Ca2+ signaling mechanism during heterocyst differentiation.
authors.affiliation: Beijing Nuclear Magnetic Resonance Center, State Key Laboratory of Plant and Protein Engineering, College of Life Sciences, Peking University, Beijing, China.
authors.forename: Yunfei
authors.initials: Y
authors.lastname: Hu
authors.forename: Xinxin
authors.initials: X
authors.lastname: Zhang
authors.forename: Yunming
authors.initials: Y
authors.lastname: Shi
authors.forename: Yanfeng
authors.initials: Y
authors.lastname: Zhou
authors.forename: Wei
authors.initials: W
authors.lastname: Zhang
authors.forename: Xiao-Dong
authors.initials: XD
authors.lastname: Su
authors.forename: Bin
authors.initials: B
authors.lastname: Xia
authors.forename: Jindong
authors.initials: J
authors.lastname: Zhao
authors.forename: Changwen
authors.initials: C
authors.lastname: Jin
mesh_terms: D000595:Amino Acid Sequence; D017033:Anabaena / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D002454:Cell Differentiation; D015964:Gene Expression Regulation, Bacterial; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D015398:Signal Transduction
chemical_list: D001426:Bacterial Proteins; D002135:Calcium-Binding Proteins; D002118:Calcium