ABSTRACT: Rod-shaped Myxococcus xanthus cells move across surfaces with defined front-rear polarity using type IVa pili (T4aP)-dependent motility and gliding. The M. xanthus genome encodes 15 PATAN domain proteins, including SgnC, SgmC, and PglH, which have been implicated in motility. Notably, six of these proteins contain a MshEN_N c-di-GMP binding motif as an α-clip insertion within the PATAN domain. Given the role of c-di-GMP in motility of M. xanthus, we characterized these proteins and found that MXAN3211, MXAN5196, and SgnC are important for both motility systems of M. xanthus, while MXAN4739 is specifically required for only T4aP-dependent motility. Next, we further investigated the molecular role of SgnC in motility of M. xanthus and found that SgnC inhibits cellular reversals in a Frz chemosensory system-dependent manner. This inhibition is independent of c-di-GMP binding of SgnC and not related to changes in the accumulation of motility-associated proteins. Surprisingly, SgnC was found to be close to the proteins that regulate T4aP formation, such as SgmX, FrzS, and SopA as revealed by our proximity labeling experiments. Consistently, our data support a direct interaction between SgnC and SgmX in vivo. Localization studies confirmed the previous findings regarding MglA, SgmX, and FrzS polar localization and demonstrated that SgnC affects the polar localization of SgmX directly, as well as MglA and PilB through its potential interaction with SgmX. In turn, proper SgmX localization is essential for SgnC polar localization. Importantly, our data suggest that SgnC and SgmX reduce sensitivity to Frz chemosensory signaling by two distinct mechanisms. First, SgnC and SgmX stimulate MglA localization at the leading cell pole, which potentially shields MglA-GTP from the Frz signaling. Second, SgnC appears to inhibit the polar binding of the Frz effector FrzZ at the leading cell pole via an unknown mechanism. Together, our findings highlight SgnC as a novel regulator of M. xanthus motility, linking chemosensory signaling to T4aP function.
Timo Glatter 