The Type IX Secretion System (T9SS) of Porphyromonas gingivalis exports proteins across the outer membrane (OM) which undergo post-translational modification (PTM) with anionic lipopolysaccharide (A-LPS) and are attached to the OM. The secreted proteins have a carboxy-terminal domain (CTD) essential for type IX secretion that is cleaved upon export. Inactivation of pg1058 encoding PG1058, a candidate T9SS component, prevented type IX substrate secretion however A-LPS was still produced and localised to the cell surface. Thus, it was demonstrated that PG1058 was an essential component of the T9SS in P. gingivalis. This study established that PG1058 is an OM-associated periplasmic protein via TEM and immunoblot analyses. Several proteins crucial to the T9SS and PTM processes have been identified in the OM including PorT (function unknown), LptO which functions in the O-deacylation of A-LPS and PorU the CTD peptidase that is itself a substrate of the T9SS. Potential interaction between PG1058 and other T9SS components required elucidation. Increased abundance of PorT and LptO in the pg1058- mutant, along with several other T9SS components, was demonstrated by immunoblot and proteomics analyses however sub-cellular localisation of these proteins did not appear altered compared to wild-type. PorU was found to accumulate in the periplasm with other T9SS substrates. No significant up-regulation of lptO, porT or porU transcription was seen in the pg1058- mutant by RT-PCR suggesting the increased protein abundance was due to increased translation or accumulation. Furthermore, the localisation and abundance of PG1058 in lptO-, porT- and porU- mutants was not altered. Together these data suggest that PG1058 is not required for the targeting of PorT and LptO to the correct cellular locale and the converse. The increased abundance of T9SS components in the absence of PG1058 confirms the involvement of PG1058 in the T9SS of P. gingivalis.