Navigating the structure–function–evolutionary relationship of CsaA chaperone in archaea

CsaA is a protein involved in the post-translational translocation of proteins across the cytoplasmic membrane. It is considered to be a functional homolog of SecB which participates in the Sec-dependent translocation pathway in an analogous manner. CsaA has also been reported to act as a molecular chaperone, preventing aggregation of unfolded proteins. It is essentially a prokaryotic protein which is absent in eukaryotes, but found extensively in bacteria and earlier thought to be widely present in archaea. The study of phylogenetic distribution of CsaA among prokaryotes suggests that it is present only in few archaeal organisms, mainly species of Thermoplasmatales and Halobacteriales. Interestingly, the CsaA protein from these two archaeal orders cluster separately on the phylogenetic tree with CsaA from Gram-positive and Gram-negative bacteria. It, thus, appears that this protein might have been acquired in these archaeal organisms through independent horizontal gene transfer (HGT) events from different bacteria. In this review, we summarize the earlier biochemical, structural, and functional characterization studies of CsaA. We draw new insights into the evolutionary history of this protein through phylogenetic and structural comparison of bacterial CsaA with modelled archaeal CsaA from Picrophilus torridus and Natrialba magadii.