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The membrane-anchored c-Src tyrosine kinase mediates signaling from a wide range of cell surface receptors controlling cell growth, adhesion, and survival. c-Src deregulation is associated with cancer. Dimerization appears to be a novel layer of regulation through a yet unclear mechanism. Binding of c-Src tyrosine kinase to the plasma membrane is mediated by the myristoylated and strongly positively charged N-terminal SH4 domain. Although activation of c-Src is known to require phosphorylation by a second c-Src molecule, electrostatic repulsion between the charged residues was considered to prevent dimerization. Here we show that a cluster of positively charged lysine residues in c-Src SH4 domain not only does not prevent dimerization but, in fact, enhances it through a lipid-mediated process. Dimerization not only depends on the number of positive charges but also on their position and the nature of the charged residues. Replacement of lysine by arginine increases dimerization in vitro and in vivo and, in HEK293T cells, causes a two-fold increase in tyrosine phosphorylation. Lipid mediated protein-protein interactions induced by clusters of basic residues may represent a general mechanism for modulating cell signaling, consistent with the abundance of positively charged residues in the juxta membrane region of many signaling proteins.

Secretory trafficking from the endoplasmic reticulum (ER) is subject to regulation by extrinsic and intrinsic factors. While much of the focus has been on biochemical triggers, little is known whether and how the ER is subject to regulation by mechanical signals. Here, we show that COPII-dependent ER-export is regulated by mechanical strain. Mechanotransduction to the ER was mediated via a previously unappreciated ER-localized pool of the small GTPase Rac1. Mechanistically, we show that Rac1 interacts with the small GTPase Sar1 to drive budding of COPII carriers and stimulate ER-to-Golgi transport. Altogether, we establish an unprecedented link between mechanical strain and export from the ER.

As cells migrate and experience forces from their surroundings, they constantly undergo mechanical deformations which reshape their plasma membrane (PM). To maintain homeostasis, cells need to detect and restore such changes, not only in terms of overall PM area and tension as previously described, but also in terms of local, nano-scale topography. Here we describe a novel phenomenon, by which cells sense and restore mechanically induced PM nano-scale deformations. We show that cell stretch and subsequent compression reshape the PM in a way that generates local membrane evaginations in the 100 nm scale. These evaginations are recognized by the I-BAR protein IRSp53, which triggers a burst of actin polymerization mediated by Rac1 and Arp2/3. The actin polymerization burst subsequently re-flattens the evagination, completing the mechanochemical feedback loop. Our results demonstrate a new mechanosensing mechanism for PM shape homeostasis, with potential applicability in different physiological scenarios.TeaserCell stretch cycles generate PM evaginations of ≈100 nm which are sensed by IRSp53, triggering a local event of actin polymerization that flattens and recovers PM shape.