PORP

PORP stands for PorP family protein, a small outer membrane lipoprotein found in many Bacteroidetes species. The term originated from porP, a gene commonly used to designate this protein in Porphyromonas gingivalis, a periodontal pathogen, but homologs are present across the phylum.

PORP is associated with the type IX secretion system (T9SS) or a closely related secretion pathway. It is believed to participate in the assembly, stabilization, or function of the secretion machinery and to contribute to the proper handling of CTD-containing cargo proteins that are exported to the cell surface. In Porphyromonas species, disruption of porP often causes defects in secretion and mislocalization of CTD proteins, including virulence factors such as gingipains, which can reduce proteolytic activity and attenuate virulence.

Structurally, PORP proteins are lipoproteins anchored to the outer membrane via an N-terminal signal peptide containing a lipobox. They are typically small, in the range of roughly 15–20 kDa, and generally lack obvious catalytic motifs. The PORP genes are usually found in operons with other T9SS components, such as porK, porL, porM, porN, porT, porU, and porV, though gene organization varies among species.

Functionally, PORP may act as a docking or stabilizing factor for CTD substrates or help connect the inner membrane secretion apparatus to surface-exposed cargo. While the exact molecular interactions remain under investigation, PORP is widely regarded as a component or accessory factor of the T9SS apparatus, contributing to efficient secretion and proper localization of CTD-containing proteins and, by extension, to the biology and pathogenic potential of several Bacteroidetes.