Stepping stone: a cytohesin adaptor for membrane cytoskeleton restraint in the syncytial Drosophila embryo.
Bottom Line: Elevating Sstn furrow levels had no effect on the steppke phenotype, but elevating Steppke furrow levels reversed the sstn phenotype, suggesting that Steppke acts downstream of Sstn and that additional mechanisms can recruit Steppke to furrows.Finally, the coiled-coil domain of Steppke was required for Sstn binding and in addition homodimerization, and its removal disrupted Steppke furrow localization and activity in vivo.Overall we propose that Sstn acts as a cytohesin adaptor that promotes Steppke activity for localized membrane cytoskeleton restraint in the syncytial Drosophila embryo.
Affiliation: Department of Cell and Systems Biology, University of Toronto, Toronto, ON M5S 3G5, Canada.Show MeSH
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Mentions: In the early Drosophila embryo, our data suggest that Sstn supports the activity of the cytohesin homologue Step for the local control of plasma membrane growth. Specifically, Sstn localizes at the base of both pseudocleavage and cellularization furrows, where it appears to engage Step through direct interactions to keep the membrane cytoskeleton in check. Without Sstn, the basal tips of the furrows expand perpendicularly in a Rho1-dependent process that leads to abnormal plasma membrane encroachment into space normally occupied by nuclei of the forming cells. This misregulation is strikingly similar to that of step loss-of-function embryos (Lee and Harris, 2013). Moreover, it could be overcome by Step overexpression, suggesting that Sstn normally acts by enhancing the activity of a limited supply of endogenous Step. Thus we propose a model in which a Sstn–Step–Arf small G protein axis acts at the base of furrows to control their growth (Figure 10). Within this axis, Sstn and Step interact directly through their CC domains, but each also has independent interactions with other membrane components, mediated by the CR in the case of Sstn. Our data suggest that the Sstn–Step interaction may stabilize Step furrow localization, where Step has been shown to use its Arf-GEF activity to control membrane growth (Lee and Harris, 2013).
Affiliation: Department of Cell and Systems Biology, University of Toronto, Toronto, ON M5S 3G5, Canada.