Protein Substrates of a Novel Secretion System Are Numerous in the Bacteroidetes Phylum and Have in Common a Cleavable C‑Terminal Secretion Signal, Extensive Post-Translational Modification, and Cell-Surface Attachment

The secretion of certain proteins in <i>Porphyromonas gingivalis</i> is dependent on a C-terminal domain (CTD). After secretion, the CTD is cleaved prior to extensive modification of the mature protein, probably with lipopolysaccharide, therefore enabling attachment to the cell surface. In this study, bioinformatic analyses of the CTD demonstrated the presence of three conserved sequence motifs. These motifs were used to construct Hidden Markov Models (HMMs) that predicted 663 CTD-containing proteins in 21 fully sequenced species of the <i>Bacteroidetes</i> phylum, while no CTD-containing proteins were predicted in species outside this phylum. Further HMM searching of <i>Cytophaga hutchinsonii</i> led to a total of 171 predicted CTD proteins in that organism alone. Proteomic analyses of membrane fractions and culture fluid derived from <i>P. gingivalis</i> and four other species containing predicted CTDs (<i>Parabacteroides distasonis</i>, <i>Prevotella intermedia</i>, <i>Tannerella forsythia</i>, and <i>C. hutchinsonii</i>) demonstrated that membrane localization, extensive post-translational modification, and CTD-cleavage were conserved features of the secretion system. The CTD cleavage site of 10 different proteins from 3 different species was determined and found to be similar to the cleavage site previously determined in <i>P. gingivalis</i>, suggesting that homologues of the C-terminal signal peptidase (PG0026) are responsible for the cleavage in these species.