Top-down Mass Spectrometry Analysis of Membrane-bound Light-Harvesting Complex 2 from Rhodobacter sphaeroides

摘要

We report a top-down proteomic analysis of the membrane-bound peripheral light-harvesting complex LH2 isolated from the purple photosynthetic bacterium Rhodobacter (Rb.) sphaeroides. The LH2 complex is coded for by the puc operon. The Rb. sphaeroides genome contains two puc operons, designated puc1BAC and puc2BA. Although previous work has shown consistently that the LH2 β polypeptide coded by the puc2B gene was assembled into LH2 complexes, there are contradictory reports whether the Puc2A polypeptides are incorporated into LH2 complexes. Furthermore, post-translational modifications (PTM) of this protein offer the prospect that it could coordinate bacteriochlorophyll a (Bchl a) by a modified N-terminal residue. Here we describe the LH2-complex components on the basis of electron-capture dissociation (ECD) fragmentation to confirm the identity and sequence of the protein subunits. We found that both gene products of the β polypeptides are expressed and assembled in the mature LH2 complex, but only the Puc1A-encoded polypeptide α is observed here. The methionine of the Puc2B-encoded polypeptide is missing, and a carboxyl group is attached to the threonine at the N terminus. Surprisingly, one amino acid encoded as an isoleucine in both the puc2B gene and the mRNA is found as valine in the mature LH2 complex, suggesting an unexpected and unusual post-translational modification or a specific tRNA recoding of this one amino acid.