Requirement of NMB0065 for connecting assembly and export of sialic acid capsular polysaccharides in Neisseria meningitidis.

Capsule expression in Neisseria meningitidis is encoded by the cps locus comprised of genes required for biosynthesis and surface translocation. Located adjacent to the gene encoding the polysialyltransferase in serogroups expressing sialic acid-containing capsule, NMB0065 is likely a member of the cps locus, but it is not found in serogroups A or X that express non-sialic acid capsules. To further understand its role in CPS expression, NMB0065 mutants were created in the serogroups B, C and Y strains. The mutants were as sensitive as unencapsulated strains to killing by normal human serum, despite producing near wild-type levels of CPS. Absence of surface expression of capsule was suggested by increased surface hydrophobicity and confirmed by immunogold electron microscopy, which revealed the presence of large vacuoles containing CPS within the cell. GC-MS and NMR analyses of purified capsule from the mutant revealed no apparent changes in polymer structures and lipid anchors. Mutants of NMB0065 homologues in other sialic acid CPS expressing meningococcal serogroups had similar phenotypes. Thus, NMB0065 (CtrG) is not involved in biosynthesis or lipidation of sialic acid-containing capsule but encodes a protein required for proper coupling of the assembly complex to the membrane transport complex allowing surface expression of CPS.