SgnC, a PATAN-domain containing response regulator functions as a noise-reducing filterin Myxococcus xanthus motility
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Philipps-Universität Marburg
Abstract
Rod-shaped Myxococcus xanthus cells move across surfaces with defined front-rear polarity using type IVa pili (T4aP)-dependent motility and gliding. This front-rear polarity in M. xanthus is maintained by polarity module proteins. Ras-like GTPase MglA is the output of this module, and in its GTP-bound form stimulates the formation of motility machineries at the leading cell pole by interacting with its effector proteins. The front-rear polarity established by the polarity module is switched occasionally during cellular reversals, causing cells to move in the opposite direction. These cellular reversals are induced by a signal from the Frz chemosensory system, and invert the localization pattern of the polarity module proteins. The molecular mechanism of this switch and overall the regulation of motility is not fully understood.
In this study, we aimed to identify additional proteins involved in the regulation of motility in M. xanthus. Particularly, we focused on PATAN domain-containing proteins, since PATAN domain is recently identified as the principal output domain binding directly to the T4aP extension motor to regulate motility in Synechocystis species. The M. xanthus genome encodes 15 PATAN domain proteins, including SgnC, SgmC, and PglH, all of which have been implicated in motility. In contrast to the Synechocystis proteins, six of the 15 PATAN domain proteins in M. xanthus contain an α-clip insertion within the PATAN domain that forms a MshEN_N c-di-GMP binding motif. As c-di-GMP is connected to the regulation of T4aP-dependent motility in M. xanthus, we investigated the role of these six PATAN domain proteins with MshEN_N insertion in regulation of motility in M.xanthus using a candidate approach. Notably, we characterized that SgnC is important for both motility systems of M. xanthus.
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 by SgnC and does not involve 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 proximity labeling experiments. Consistently, our data support a direct interaction between SgnC and SgmX in vivo. Localization studies demonstrated that SgnC dynamically localizes to the leading cell pole, and is recruited to this pole by SgmX through direct interaction. In turn, SgnC stimulates SgmX polar localization directly, resulting in increased MglA and PilB localization at the leading cell pole. Therefore, MglA/SgmX/SgnC
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establishes positive feedback that reinforces their polar localization, as well as PilB polar localization and T4aP extension. Our localization studies also demonstrate that SgnC inhibits the polar localization of Frz chemosensory signaling effector FrzZ-P to the leading cell pole, which was shown to be recruited to the leading cell pole in a MglA-dependent manner.
Importantly, our data suggest that SgnC reduces sensitivity to Frz chemosensory signaling by two distinct mechanisms. First, SgnC inhibits the polar binding of the FrzZ-P at the leading cell pole via an unknown mechanism and this inhibition overcomes the stimulation of FrzZ-P polar localization by MglA. Second, by establishing the MglA/SgmX/SgnC positive feedback that reinforces the polar localization of MglA, SgnC would make the polarity module more resistant to Frz signaling.