Cell patterning by secretion-induced plasma membrane flows

Cells self-organize using reaction-diffusion and fluid-flow principles. Whether bulk membrane flows contribute to cell patterning has not been established. Here, using mathematical modelling, optogenetics and synthetic probes, we show that polarized exocytosis causes lateral membrane flows away from regions of membrane insertion. Plasma membrane-associated proteins with sufficiently low diffusion and/or detachment rates couple to the flows and deplete from areas of exocytosis. In rod-shaped fission yeast cells, zones of Cdc42 GTPase activity driving polarized exocytosis are limited by GTPase activating proteins (GAPs). We show that membrane flows pattern the GAP Rga4 distribution and coupling of a synthetic GAP to membrane flows is sufficient to establish the rod shape. Thus, membrane flows induced by Cdc42-dependent exocytosis form a negative feedback restricting the zone of Cdc42 activity.One Sentence SummaryExocytosis causes bulk membrane flows that drag associated proteins and form a negative feedback restricting the exocytic site.

GIV/Girdin and Exo70 Constitute the Core of the Mammalian Polarized Exocytic Machinery

SUMMARYPolarized exocytosis is a fundamental process by which membrane and cargo proteins are delivered to the plasma membrane with precise spatial control; it is essential for cell growth, morphogenesis, and migration. Although the need for the octameric exocyst complex is conserved from yeast to humans, what imparts spatial control is known only in yeast, i.e., a polarity scaffold without mammalian homolog, called Bem1p. We demonstrate that polarity scaffold GIV/Girdin fulfills the key criteria and functions of its yeast counterpart Bem1p. Both Bem1p and GIV bind yeast and mammalian Exo70 proteins via similar short-linear interaction motifs, but each preferentially binds its evolutionary counterpart. In cells where this GIV•Exo-70 interaction is selectively disrupted, delivery of the metalloprotease MT1-MMP to podosomes, collagen degradation and haptotaxis through basement membrane matrix were impaired. GIV’s interacting partners reveal other components of polarized exocytosis in mammals. Findings not only expose how GIV “upgrades” the exocytic process in mammals, but also how the ability to regulate exocytosis shapes GIV’s ability to fuel metastasis.GRAPHIC ABSTRACTGraphic Abstract: Schematic comparing the components of polarized exocytosis, i.e., the major polarity scaffold in yeast (Bem1p; left) and humans (Girdin; right) and the various cellular components and signaling mechanisms that are known to converge on them.The eTOC blurbPolarized exocytosis is a precision-controlled process that is enhanced in disease states, e.g., cancer invasion; what imparts polarity was unknown. Authors reveal how the process underwent an evolutionary upgrade from yeast to humans by pinpointing GIV/Girdin as the polarity scaffold which orchestrates the exocytosis of matrix metalloproteases during cell invasion.HIGHLIGHTSGIV (human) and Bem1p (yeast) bind Exo70; are required for exocytosisGIV binds and aids PM localization Exo70 via a conserved short linear motifBinding facilitates MT1-MMP delivery to podosomes, ECM degradation, invasionRegulatory control over polarized exocytosis is upgraded during evolution

Time-gated confocal microscopy reveals accumulation of exocyst subunits at the plant-pathogen interface

Polarized exocytosis is essential for plant development and defence. The exocyst, an octameric protein complex that tethers exocytotic vesicles to the plasma membrane, targets exocytosis. Upon pathogen attack, secreted materials form papillae to halt pathogen penetration. To determine if the exocyst is directly involved in targeting exocytosis to infection sites, information about its localization is instrumental. Here, we investigated exocyst subunit localization in the moss Physcomitrella patens upon pathogen attack and infection by Phytophthora capsici. Time-gated confocal microscopy was used to eliminate autofluorescence of deposited material around infection sites allowing the visualization of the subcellular localization of exocyst subunits and of v-SNARE Vamp72A1-labeled exocytotic vesicles during infection. This showed that exocyst subunits Sec3a, Sec5b, Sec5d and Sec6 accumulated at sites of attempted pathogen penetration. Upon pathogen invasion, the exocyst subunits accumulated on the membrane surrounding papilla-like structures and hyphal encasements. Vamp72A1-labeled vesicles were found to localize in the cytoplasm around infection sites. The re-localization of exocyst subunits to infection sites suggests that the exocyst is directly involved in facilitating polarized exocytosis during pathogenesis.

A Presenilin-2–ARF4 trafficking axis modulates Notch signaling during epidermal differentiation

How primary cilia impact epidermal growth and differentiation during embryogenesis is poorly understood. Here, we show that during skin development, Notch signaling occurs within the ciliated, differentiating cells of the first few suprabasal epidermal layers. Moreover, both Notch signaling and cilia disappear in the upper layers, where key ciliary proteins distribute to cell–cell borders. Extending this correlation, we find that Presenilin-2 localizes to basal bodies/cilia through a conserved VxPx motif. When this motif is mutated, a GFP-tagged Presenilin-2 still localizes to intercellular borders, but basal body localization is lost. Notably, in contrast to wild type, this mutant fails to rescue epidermal differentiation defects seen upon Psen1 and 2 knockdown. Screening components implicated in ciliary targeting and polarized exocytosis, we provide evidence that the small GTPase ARF4 is required for Presenilin basal body localization, Notch signaling, and subsequent epidermal differentiation. Collectively, our findings raise the possibility that ARF4-dependent polarized exocytosis acts through the basal body–ciliary complex to spatially regulate Notch signaling during epidermal differentiation.