Moleulcar characterisation of lipoprotein processing in streptococcus uberis

Streptococcus uberis is a common cause of bovine mastitis and the lipoprotein MtuA has been shown to be essential for growth in milk and virulence. The enzymes responsible for processing lipoproteins in other Gram positive bacteria are lipoprotein diacylglyceryl transferase (Lgt) which acts to anchor lipoproteins to the membrane and lipoprotein signal peptidase (Lsp) which cleaves the signal peptide. S. uberis mutants containing lesions in lgt and lsp uncovered several novel phenotypes. A number of additional proteins were shown to be present in extracellular fractions prepared from lgt- and lgt-/lsp- mutants when compared to the equivalent fraction prepared from wild type bacteria. Atypical processing of MtuA and other lipoproteins within the signal peptide was shown to occur, indicating the presence of an activity capable of shaving such proteins from the membrane in the absence of Lgt and Lsp activity. MtuA was shown to be released into the extracellular space by Wetern blot; the size closely resembled that of wild type protein in both the lgt- and lgt-/lsp- mutants. A metallopeptidase that can be inhibited by phosphoramidon and metal ion chelating agents may be responsible for the activity that results in these proteins being alternatively processed. MtuA in the lsp- mutant had a molecular weight that corresponded to full length MtuA and remained localised in the membrane as seen in the wild type. During late log phase a second form of MtuA with a lower molecular weight was detected. A mutant containing insertions in both lsp and the gene encoding the S. uberis homologue to the Enterococcus faecalis was studied. Enhanced expression of pheromone (eep) suggested that this metallopeptidase was also able to cleave the signal peptide of MtuA.