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).

*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)
References:
Anand A., LeDoyt M., Karanian C., Luthra A., Koszelak-Rosenblum M., Malkowski MG., Puthenveetil R., Vinogradova O. and Radolf JD. (2015). Bipartite Topology of Treponema pallidum Repeat Proteins C/D and I: OUTER MEMBRANE INSERTION, TRIMERIZATION, AND PORIN FUNCTION REQUIRE A C-TERMINAL beta-BARREL DOMAIN. J Biol Chem. 290(19):12313-31.
Anand, A., A. Luthra, M.E. Edmond, M. Ledoyt, M.J. Caimano, and J.D. Radolf. (2013). The major outer sheath protein (Msp) of Treponema denticola has a bipartite domain architecture and exists as periplasmic and outer membrane-spanning conformers. J. Bacteriol. 195: 2060-2071.
Cameron, E.A., M.A. Maynard, C.J. Smith, T.J. Smith, N.M. Koropatkin, and E.C. Martens. (2012). Multidomain Carbohydrate-binding Proteins Involved in Bacteroides thetaiotaomicron Starch Metabolism. J. Biol. Chem. 287: 34614-34625.
Cho, K.H. and A.A. Salyers. (2001). Biochemical analysis of interactions between outer membrane proteins that contribute to starch utilization by Bacteroides thetaiotaomicron. J. Bacteriol. 183: 7224-7230.
Egli, C., W.K. Leung, K.H. Muller, R.E. Hancock, and B.C. McBride. (1993). Pore-forming properties of the major 53-kilodalton surface antigen from the outer sheath of Treponema denticola. Infect. Immun. 61: 1694-1699.
Fenno, J.C., P.M. Hannam, W.K. Leung, M. Tamura, V.J. Uitto, and B.C. McBride. (1998). Cytopathic effects of the major surface protein and the chymotrypsinlike protease of Treponema denticola. Infect. Immun. 66: 1869-1877.
Foley, M.H., E.C. Martens, and N.M. Koropatkin. (2018). SusE facilitates starch uptake independent of starch binding in B. thetaiotaomicron. Mol. Microbiol. 108: 551-566.
Giacani, L., S.L. Brandt, M. Puray-Chavez, T.B. Reid, C. Godornes, B.J. Molini, M. Benzler, J.S. Hartig, S.A. Lukehart, and A. Centurion-Lara. (2012). Comparative investigation of the genomic regions involved in antigenic variation of the TprK antigen among treponemal species, subspecies, and strains. J. Bacteriol. 194: 4208-4225.
Godovikova, V., M.P. Goetting-Minesky, and J.C. Fenno. (2011). Composition and localization of Treponema denticola outer membrane complexes. Infect. Immun. 79: 4868-4875.
Godovikova, V., M.P. Goetting-Minesky, J.C. Timm, and J.C. Fenno. (2019). Immunotopological Analysis of the Major Surface Protein (Msp). J. Bacteriol. 201:.
Goetting-Minesky, M.P., V. Godovikova, W. Zheng, and J.C. Fenno. (2022). Characterization of Treponema denticola Major Surface Protein (Msp) by Deletion Analysis and Advanced Molecular Modeling. J. Bacteriol. 204: e0022822.
Joglekar, P., E.D. Sonnenburg, S.K. Higginbottom, K.A. Earle, C. Morland, S. Shapiro-Ward, D.N. Bolam, and J.L. Sonnenburg. (2018). Genetic Variation of the SusC/SusD Homologs from a Polysaccharide Utilization Locus Underlies Divergent Fructan Specificities and Functional Adaptation in Strains. mSphere 3:.
Kumar, S., M.J. Caimano, A. Anand, A. Dey, K.L. Hawley, M.E. LeDoyt, C.J. La Vake, A.R. Cruz, L.G. Ramirez, L. Paštěková, I. Bezsonova, D. Šmajs, J.C. Salazar, and J.D. Radolf. (2018). Sequence Variation of Rare Outer Membrane Protein β-Barrel Domains in Clinical Strains Provides Insights into the Evolution of subsp. , the Syphilis Spirochete. MBio 9:.
Mathers, D.A., W.K. Leung, J.C. Fenno, Y. Hong, and B.C. McBride. (1996). The major surface protein complex of Treponema denticola depolarizes and induces ion channels in HeLa cell membranes. Infect. Immun. 64: 2904-2910.
Puthenveetil, R., S. Kumar, M.J. Caimano, A. Dey, A. Anand, O. Vinogradova, and J.D. Radolf. (2017). The major outer sheath protein forms distinct conformers and multimeric complexes in the outer membrane and periplasm of Treponema denticola. Sci Rep 7: 13260.
Reid, T.B., B.J. Molini, M.C. Fernandez, and S.A. Lukehart. (2014). Antigenic variation of TprK facilitates development of secondary syphilis. Infect. Immun. 82: 4959-4967.
Shipman, J.A., J.E. Berleman, and A.A. Salyers. (2000). Characterization of four outer membrane proteins involved in binding starch to the cell surface of Bacteroides thetaiotaomicron. J. Bacteriol. 182: 5365-5372.
Examples:
TC#	Name	Organismal Type	Example
1.B.38.1.1	Large non-selective MSP porin of 574 aas, with short lived large ion conduction (Mathers et al. 1996). Contains MOSP_N and MOSP_C domains which exists as periplasmic hydrophilic monomers and trimeric porins, respectively. MOSP_C, destined for the OM, follows the canonical BAM pathway, but formation of a stable periplasmic conformer of MOSP_N involves an export-related, folding pathway not present in E. coli (Puthenveetil et al. 2017).	*Spirochetes (Treponema* species)**	Msp of Treponema denticola

1.B.38.1.10	*SusD protein of 570 aas and 1 N-terminal TMS (Joglekar et al.* 2018).**		SusD of Bacteroides thetaiotaomicron

1.B.38.1.2	Treponema repeat protein K (TprK), an outer membrane surface exposed variable antigen which plays a role in immune evasion and persistence (Giacani et al. 2012; Reid et al. 2014).	Spirochaetes	TprK of Treponema pallidum

1.B.38.1.3	Repeat protein, TprEb	Spirochaetes	Repeat protein, TprEb

1.B.38.1.4	Major outer membrane sheath protein, Msp or MOSP, of 543 aas. Msp has a bipartide structure and exists as periplasmic and outer membrane-integrated trimeric conformers (Anand et al. 2013). The N-terminal domain (residues 77 - 286) does not insert into the membrane, but the C-terminal domain (residues 332 - 543) does to form pores (Anand et al. 2013). It resembles the surface exposed variable antigen, TprK (Giacani et al. 2012) which plays roles in immune evasion and persistence. MOSP is one of its principal cell surface virulence determinants. Bioinformatics predicts that MOSP consists of N- and C-terminal domains, MOSP^N and MOSP^C. Biophysical analysis of constructs refolded in vitro demonstrated that MOSP^C, which has porin activity, forms amphiphilic trimers, while MOSP^N forms an extended hydrophilic monomer (Puthenveetil et al. 2017). It is also a constituent of the outer membrane lipoprotein-protease complex of the Dentilisin Family (see TC# 9.B.355.1.1). Msp has been characterized by deletion analysis and advanced molecular modeling (Goetting-Minesky et al. 2022). It is a large-diameter, trimeric outer membrane porin-like protein.	Spirochaeta	Major outer sheath protein, Msp, of Treponema denticola

1.B.38.1.5	Outer membrane bipartite trimeric porin of 598 aas with an N-terminal MOSPN domain (in the periplasm), and a C-terminal MOSPC domain (cell surface localized), TprC/TprD. The MOSPN domain confers envelope integrity by anchoring the C-terminal porin domain to periplasmic structural constituents (Anand et al. 2015). Selection pressures exerted within human populations drive T. pallidum subsp. pallidum TrpC diversity by mutation of loop regions and by recombination(Kumar et al. 2018).	Spirochaetes	TprC of Treponema pallidum

1.B.38.1.6	*Outer membrane trimeric porin, TprI of 598 aas with a structure similar to that of TprC (TC# 1.B.38.1.5) (Anand et al.* 2015).**	Spirochaetes	TprI of Treponema pallidum

1.B.38.1.7	SusD of 502 aas and 1 N-terminal TMS.		SusD of Bacteroides fragilis

1.B.38.1.8	Uncharacterized major outer membrane protein of 511 aas and 1 N-terminal TMS.		UP of Treponema vincentii

1.B.38.1.9	Outer membrane pore-forming TprA protein of 607 aas. This protein is 91% identical to the TprA protein of T. pallidum.		TprA of Treponema paraluiscuniculi

Examples:
TC#	Name	Organismal Type	Example
1.B.38.2.1	Putative outer membrane protein of 460 aas. Shows some sequence similarity to autotransporters (1.B.40)	Spirochaetes	OMP of Spirochaeta thermophila

1.B.38.2.2	Putative outer membrane protein of 446 aas. Shows some sequence similiarity to autotransporters (1.B.40)	Spirochaetes	OMP of Spirochaeta thermophila

1.B.38.2.3	Uncharacterized porin of 389 aas and 1 N-terminal TMS.		UP of Spirochaetae bacterium

1.B.38.2.4	Uncharacterized protein of 469 aas.		UP of Spirochaeta perfilievii

Examples:
TC#	Name	Organismal Type	Example
1.B.38.3.1	Putative sheath protein of 520 aas with 14 putative β-TMSs at the N-terminus and a fairly long C-terminal extension.	Spirochaetes	Putative sheath protein of Treponema brennaborense

1.B.38.3.2	Putative outer membrane protein of 575 aas and 24 putative β-TMSs, MspA	Spirochaetes	MspA of Treponema maltophilum

1.B.38.3.3	Putative outer membrane protein of 590 aas and 22 putative β-TMSs.	Spirochaetes	OMP of Treponema lecithinolyticum

Examples:
TC#	Name	Organismal Type	Example
1.B.38.4.1	Outer membrane maltooligosaccharide uptake protein, SusE, of 387 aas and 1 N-terminal TMS. It forms a complex with the SusC porin (TC# 1.B.14.6.1), the SusD porin (TC# 1.B.38.1.10), the SusF porin (TC# 1.B.38.4.2) and SusG (α-amylase; TC# 8.A.9.1.3) in the outer membrane (Foley et al. 2018). The complex binds starch and maltooligosaccharides (Cho and Salyers 2001).		OMP of Bacteroides thetaiotaomicron

1.B.38.4.2	Outer membrane protein, SusF, of 485 aas and 1 N-terminal TMS. The protein has an N-terminal DUF5115 domain followed by two C-terminal CBM-SusEF-like domains. SusF mediates starch-binding (or maltooligosaccharde-binding) before transport into the periplasm for further degradation. SusE and SusF do not constitute the major starch-binding proteins in the starch degradative pathway. SusF has lower affinity for starch compared to SusE (Shipman et al. 2000). The 3-d structure of the complex has been determined (Cameron et al. 2012). The SusCDEFG complex in the outer membrane is described in more detail in TC# 1.B.38.4.1 (Foley et al. 2018).		OMP, SusF, of Bacteroides thetaiotaomicron

1.B.38.4.3	SusE/F homologue of 347 aas and 1 N-terminal TMS.		AusE of Pontibacter lucknowensis

1.B.38.4.4	Uncharacterized DUF5115 domain-containing protein of 477 aas and 1 N-terminal TMS.		UP of Chryseobacterium chaponense

1.B.38.4.5	Uncharacterized SusD homologue of 531 aas and 1 N-terminal TMS.		SusD homologue of Phaeodactylibacter xiamenensis