There's no place like OM: subcellular sorting and secretion of PPAD in Porphyromonas gingivalis

The oral pathogen Porphyromonas gingivalis is one of the major periodontal agents and it has been recently hailed as a potential cause of the autoimmune disease rheumatoid arthritis. The peptidylarginine deiminase enzyme of P. gingivalis (PPAD) appears to be responsible, in genetically predisposed subjects, for the citrullination of certain host proteins and the subsequent appearance of antibodies against citrullinated proteins, which might play a role in the etiology of rheumatoid arthritis. Analysis of a large panel of P. gingivalis isolates showed that PPAD is present in almost all samples with the same expression pattern: an outer membrane (OM)-bound version, also present in outer membrane vesicles, and a secreted version. Interestingly, a small subset of isolates seems to lack the OM-bound PPAD and only possesses its secreted version. Our analyses showed that this aberrant phenotype doesn't appear to be due to higher levels of proteolytic activities or lower levels of vesicles production but most likely to changes in the amino acid sequences of the PPAD proteins expressed by these isolates.