The atypical hybrid histidine protein kinase RodK in Myxococcus xanthus: spatial proximity supersedes kinetic preference in phosphotransfer reactions.

Many proteins of two-component signal transduction systems (TCS) have domain structures that do not comply with a phosphate flow as observed in linear TCS, phosphorelays, or simple branched pathways. An example is RodK, which is essential for fruiting body formation in Myxococcus xanthus and, in addition to a sensor domain, consists of a kinase domain and three receiver domains (RodK-R1, -R2, and -R3), all of which are functionally important. We identified the RokA response regulator as part of the RodK pathway. In vitro the isolated RodK kinase domain engages in phosphotransfer to RodK-R3 and RokA, with a kinetic preference for RokA. However, in the context of the full-length protein, the RodK kinase domain has a preference for phosphotransfer to RodK-R3 over RokA. We suggest that in full-length RodK, the spatial proximity of the RodK kinase domain and RodK-R3 compensate for the kinetic preference of the isolated kinase domain for RokA. Thus, the kinetic preference observed using an isolated kinase domain of a hybrid kinase does not necessarily reflect the phosphotransfer preference of the full-length protein. We speculate that the phosphorylation status of RodK-R1 and RodK-R2 determines whether RodK engages in phosphotransfer to RodK-R3 or RokA in vivo.