| The gram-positive respiratory pathogen Streptococcus pneumoniae produces a number of proteins that are necessary for full virulence; however only one virulence factor is absolutely required, the polysaccharide capsule. While the upstream common region of the type 2 capsule locus encodes proteins thought to regulate capsule transcription or polymerization, the mechanism of this regulation is poorly understood. Recently homologues of the upstream capsule proteins, CpsC and CpsD, have been shown to be autophosphorylating tyrosine kinases. While the phosphorylation of tyrosine residues has been long established as a mechanism of regulation in eukaryotic organisms, this is a newly described method of regulation in prokaryotes. Together, Cps2C and Cps2D from serotype 2 S. pneumoniae form an active autophosphorylating tyrosine kinase. Cps2C is a transmembrane protein necessary to initially activate Cps2D, while Cps2D contains the ATP-binding domain and the phosphotyrosine acceptor domain of the functional autophosphorylating kinase. Both proteins are required for mature capsule production in S. pneumoniae and, subsequently, cps2C and cps2D deletion mutants are avirulent. Cps2B is a novel tyrosine phosphatase that also inhibits the initial phosphorylation of Cps2D. While cps2B was not essential for normal levels of capsule production in S. pneumoniae , it was required for complete virulence, as deletion mutants were reduced in their ability to kill mice intravenously when compared to their parent D39. Finally, the amount of phosphorylated Cps2D directly correlates with both the amount of capsule produced as well as protein levels of Cps2E, the first glycosyltransferase required for repeat unit biosynthesis. While transcriptional regulation may account for some of the changes in Cps2E, the severe reduction of Cps2E in the cps2D mutant cannot be explained by the reduction of transcript alone and suggests a post-transcriptional mechanism of regulation. Taken together these data suggest that tyrosine phosphorylation regulates capsule production by controlling the formation of repeat unit precursors and not polymerization as previously thought. |
