Sporulation initiation phospho-transferase B or SpoOB is part of a phospho-relay that initiates sporulation in Bacillus subtilis. Spo0B is a two-domain protein consisting of an N-terminal alpha-helical hairpin domain and a C-terminal alpha/beta domain. Two subunits of Spo0B dimerise by a parallel association of helical hairpins to form a novel four-helix bundle from which the active histidine - involved in the auto-phosphorylation - protrudes. In the phospho-relay, the signal-receptor histidine kinases are dephosphorylated by a common response regulator, Spo0F. Spo0B then takes phosphorylated Spo0F as substrate thereby mediating the transfer of a phosphoryl group to Spo0A, the ultimate transcription factor. The exact function of this alpha-helical domain is not known; it does not always occur just as the N-terminal domain of SPOB_ab, Pfam:PF14682. SCOP describes this domain as a histidine kinase-like fold lacking the kinase ATP-binding site.

1: Formation of a novel four-helix bundle and molecular recognition sites by dimerization of a response regulator phosphotransferase. Varughese KI, Madhusudan, Zhou XZ, Whiteley JM, Hoch JA; Mol Cell. 1998;2:485-493. PMID:9809070