1.B.38 The Treponema Porin Major Surface Protein (TP-MSP) Family
MSP is a 53 kDa (474 aa) surface antigen in the outer sheath of Treponema denticola. It is an adhesin, but additionally it has been purified to homogeneity and reconstituted in black lipid membranes where it showed channel activity (Egli et al., 1993). It also induces channel activity in HeLa cell membranes (Mathers et al., 1996). The channel had a single conductance of 1.8 nS in 0.1 M KCl (estimated pore diameter of 3.4 nm), the largest porin channel documented in 1996. Electron microscopy suggested a regular hexagonal array in the membrane. Homologues are found in other Treponema species.
The Treponema denticola outer membrane lipoprotein-protease complex (Dentilisin) contributes to periodontal disease by degrading extracellular matrix components and disrupting intercellular host signaling pathways (Godovikova et al. 2011). prcB, located upstream of and cotranscribed with prcA and prtP, encodes a 22-kDa lipoprotein that interacts with PrtP and is required for its activity. PrcB migrates in native gels as part of a >400-kDa complex that includes PrtP and PrcA, as well as the major outer sheath protein Msp. Though it lacks the canonical ribosome binding site present upstream of both prcA and prtP, PrcB is present at levels similar to those of PrtP in whole-cell extracts. Immunofluorescence microscopy demonstrated cell surface exposure of the mature forms of PrtP, PrcA1, PrcB, and Msp. The 16-kDa N-terminal acylated fragment of PrtP (predicted to be released during activation of PrtP) was present in cell extracts but was detected neither in the purified active protease complex nor on the cell surface. PrcA2, detectable on the surface of Msp-deficient cells but not that of wild-type cells, coimmunoprecipitated with Msp. These results indicate that PrcB is a component of the outer membrane lipoprotein protease complex and that the Msp and PrcA2 interaction may mediate formation of a very-high-molecular-weight outer membrane complex (Godovikova et al. 2011).
The major outer sheath protein (Msp or MOSP) has a bipartite domain architecture and exists as periplasmic and outer membrane-spanning conformers (Anand et al. 2013). The Msp complex depolarized and increased the conductance of the HeLa cell membrane in a manner which was not strongly selective for Na+, K+, Ca2+, and Cl- ions. Cell-attached patches of HeLa cell membrane exposed to the Msp complex exhibited short-lived channels with a slope conductance of 0.4 nS in physiologically normal saline (Mathers et al. 1996). Pore-forming activities of recombinant Msp in black lipid model membrane assays and in HeLa cell membranes were similar to those reported for the native protein, supporting the hypothesis that Msp cytotoxicity is due to its pore-forming activity (Fenno et al. 1998). The oligomeric outer membrane-associated complex binds fibronectin and disrupts several intracellular responses.It may form a large-diameter β-barrel porin (Godovikova et al. 2019).
The transport reaction catalyzed by MSP is:
ions (non-selective)(periplasm) 
ions (out)