Combinatorial control of type IVa pili formation by MglA, SgmX, FrzS and SopA in Myxococcus xanthus

Attachment to and translocation across surfaces using type IVa pili (T4aP) is of paramount importance for biofilm formation and virulence of several bacteria. T4aP are highly dynamic filaments, which undergo cycles of extension, surface adhesion, and retraction, sufficient to pull the cell forward. For the efficient translocation of rod-shaped bacteria over a surface, it is critical that T4aP are only formed at one of the poles. The rod-shaped Gram-negative soil bacterium Myxococcus xanthus moves with well-defined leading and lagging cell poles using T4aP, which are formed at the leading pole only. This localization is determined by the accumulation of MglA, a small Ras-like GTPase, and its downstream effector SgmX, a tetratricopeptide repeat-domain containing protein, at the leading pole. In this work, we addressed the underlying mechanisms of how SgmX stimulates T4aP formation. By precise quantification of the polar localization of MglA and SgmX, we confirm that MglA recruits SgmX and the pseudo-response regulator FrzS, which was also shown to be important for T4aP formation. FrzS, in turn, also stimulates polar SgmX localization. Moreover, FrzS and SgmX separately recruit the newly identified protein SopA to the leading cell pole. The phenotypic analysis of ΔsgmX, ΔfrzS, and ΔsopA mutants revealed that SgmX is essential for T4aP formation, whereas FrzS and SopA alone are not sufficient for T4aP formation. The observation that the single ΔsgmX, ΔfrzS, and ΔsopA mutants make progressively more T4aP supports that FrzS and SopA separately stimulate SgmX-dependent T4aP formation and T4aP-dependent motility, whereby the FrzS stimulation is more efficient. SgmX brings about the polar localization of the ATPase PilB, which is driving T4aP extension. However, a direct interaction between these two proteins has never been shown. We demonstrate that FrzS and SopA are also important and essential for the polar recruitment of PilB, respectively. We confirm that SgmX directly interacts with FrzS; moreover, we observed that SgmX interacts with SopA. Collectively, our data support that the main pathway for stimulating T4aP formation, and T4aP-dependent motility, is the MglA/SgmX pathway. FrzS further stimulates the pathway by recruiting SgmX and SopA. SopA, in turn, stimulates the function of SgmX, likely by promoting the SgmX-dependent interaction of the extension ATPase PilB with the base of the T4aP machine. The MglA/FrzS/SgmX/SopA pathway for orchestrating T4aP formation at the leading cell pole displays an architecture that allows for multiple layers of regulation and is reminiscent of a coincidence detector circuit. The maximal T4aP formation and T4aP-dependent motility are only accomplished when all four regulators are present.