RESUMO
The protein CsaA has been proposed to function as a protein secretion chaperone in bacteria that lack the Sec-dependent protein-targeting chaperone SecB. CsaA is a homodimer with two putative substrate-binding pockets, one in each monomer. To test the hypothesis that these cavities are indeed substrate-binding sites able to interact with other polypeptide chains, we selected a peptide that bound to CsaA from a random peptide library displayed on phage. Presented here is the structure of CsaA from Agrobacterium tumefaciens (AtCsaA) solved in the presence and absence of the selected peptide. To promote co-crystallization, the sequence for this peptide was genetically fused to the amino-terminus of AtCsaA. The resulting 1.65 A resolution crystal structure reveals that the tethered peptide from one AtCsaA molecule binds to the proposed substrate-binding pocket of a symmetry-related molecule possibly mimicking the interaction between a pre-protein substrate and CsaA. The structure shows that the peptide lies in an extended conformation with alanine, proline and glutamine side chains pointing into the binding pocket. The peptide interacts with the atoms of the AtCsaA-binding pocket via seven direct hydrogen bonds. The side chain of a conserved pocket residue, Arg76, has an "up" conformation when the CsaA-binding site is empty and a "down" conformation when the CsaA-binding site is occupied, suggesting that this residue may function to stabilize the peptide in the binding cavity. The presented aggregation assays, phage-display analysis and structural analysis are consistent with AtCsaA being a general chaperone. The properties of the proposed CsaA-binding pocket/peptide interactions are compared to those from other structurally characterized molecular chaperones.
Assuntos
Agrobacterium tumefaciens/metabolismo , Proteínas de Bactérias/química , Chaperonas Moleculares/química , Conformação Proteica , Sequência de Aminoácidos , Animais , Bacillus subtilis/química , Proteínas de Bactérias/genética , Proteínas de Bactérias/metabolismo , Sítios de Ligação , Cristalografia por Raios X , Dimerização , Ligação de Hidrogênio , Ligantes , Modelos Moleculares , Chaperonas Moleculares/genética , Chaperonas Moleculares/metabolismo , Dados de Sequência Molecular , Biblioteca de Peptídeos , Peptídeos/química , Peptídeos/genética , Peptídeos/metabolismo , Alinhamento de Sequência , Thermus thermophilus/químicaRESUMO
Bacillus subtilis CsaA (BsCsaA) has been proposed to act as a protein-secretion chaperone in the Sec-dependent translocation pathway, possibly compensating for the lack of SecB in the Gram-positive eubacterium Bacillus subtilis. This paper presents the cloning, purification, crystallization and structures of BsCsaA in two space groups (P42(1)2 and P3(2)21) solved and refined to resolutions of 1.9 and 2.0 A, respectively. These structures complement the previously available crystal structure of CsaA from the Gram-negative eubacterium Thermus thermophilus (TtCsaA) and provide a direct structural basis for the interpretation of previously available biochemical data on BsCsaA. The sequence and structure of the proposed substrate-binding pocket are analyzed and discussed. A comparison with the TtCsaA structure reveals a different pattern of electrostatic potential in the vicinity of the binding site, which overlaps with a region of high sequence variability. In addition, the dimerization interface of this homodimeric protein is analyzed and discussed.
