KapB is a lipoprotein required for KinB signal transduction and activation of the phosphorelay to sporulation in Bacillus subtilis.

KinB is one of the two major histidine kinases that provide phosphate input in the phosphorelay to produce SpoOA approximately P, the key transcription factor controlling the initiation of sporulation. A search for insertion mutants affected in activation of KinB-dependent sporulation led to the identification of the lgt locus encoding the lipoprotein glyceryltransferase required for the lipid modification of prolipoproteins before their cleavage and translocation across the cytoplasmic membrane. In parallel, a putative lipoprotein signal peptide cleavage site was detected in KapB, known to be strictly required for KinB-mediated sporulation and located downstream of KinB in a single transcription unit. Using PhoA peptide fusions, we have shown that KapB signal-peptide can direct active alkaline phosphatase to the outer surface of the cytoplasmic membrane in an LGT-dependent manner, strongly suggesting that KapB is a lipoprotein tethered to the outer face of the cytoplasmic membrane via a lipid anchor. As KapB proved to be dispensable for expression of the kinBkapB operon, a chimeric kinase was built consisting of KinA sensor domain fused to KinB kinase domain (KinA'-'B) to assess (i) the involvement of KapB in catalysis of the kinase reaction, and (ii) the ability of KinB to phosphorylate SpoOF in vitro. It was shown that KapB is dispensable for both in vivo and in vitro activation of the phosphorelay by the KinA'-'B chimera and that KinA'-'B phosphorylates SpoOF directly in vitro. Models for the role of KapB in regulating KinB activity are discussed.