Uncovering the structure-function aspects of an archaeal CsaA protein

CsaA is known to function as a protein secretion chaperone in bacteria. Homologs of CsaA are also found in archaea while they are absent in eukaryotes. This paper presents the biophysical, biochemical analysis and crystallographic structure determination of CsaA from a thermoacidophilic archaeon Picrophilus torridus (PtCsaA). The PtCsaA appears to prevent the aggregation of heat denatured Bovine Carbonic Anhydrase II (BCAII). Differential denaturation of PtCsaA by guanidine hydrochloride (Gdn-HCl) and urea indicates the stabilization of the protein via salt bridges. Denaturant mediated decrease in 8-Anilinonaphthalene-1-sulfonic acid (ANS) binding and shift in wavelength signifies the partial unfolding of the protein molecule and exposure of hydrophobic patches to solvent on denaturation. The crystal structure of PtCsaA was solved to a resolution of 1.7 Å. The structure of PtCsaA appears to be similar to bacterial CsaA in architecture. Docking of a six amino acid peptide in the substrate binding pocket of PtCsaA suggests conservation in the substrate binding cavity. Residues involved in the formation of the binding cavity and hydrogen bonds responsible for the dimerization of PtCsaA were compared with those observed in the structure of Bacillus subtilis CsaA. The similarities and differences in electrostatic surface potential of the substrate binding cavities in bacterial CsaA and PtCsaA are discussed.