Missing-in-metastasis and IRSp53 deform PI(4,5)P2-rich membranes by an inverse BAR domain–like mechanism

Abstract

The actin cytoskeleton plays a fundamental role in various motile and morphogenetic processes involving membrane dynamics. We show that actin-binding proteins MIM (missing-in-metastasis) and IRSp53 directly bind PI(4,5)P2-rich membranes and deform them into tubular structures. This activity resides in the N-terminal IRSp53/MIM domain (IMD) of these proteins, which is structurally related to membrane-tubulating BAR (Bin/amphiphysin/Rvs) domains. We found that because of a difference in the geometry of the PI(4,5)P2-binding site, IMDs induce a membrane curvature opposite that of BAR domains and deform membranes by binding to the interior of the tubule. This explains why IMD proteins induce plasma membrane protrusions rather than invaginations. We also provide evidence that the membrane-deforming activity of IMDs, instead of the previously proposed F-actin–bundling or GTPase-binding activities, is critical for the induction of the filopodia/microspikes in cultured mammalian cells. Together, these data reveal that interplay between actin dynamics and a novel membrane-deformation activity promotes cell motility and morphogenesis.

MTSS1 is a metastasis driver in a subset of human melanomas

“…While these data are supported by observations from other cell lines where ectopic expression of MTSS1 or its I-BAR domain results in reduction of stress fibers 35 , a hallmark phenotype for RhoA inactivation 62 , they do not exclude additional indirect Rac and RhoA regulation through MTSS1 by affecting the geometry or lipid distribution of the leading edge plasma membrane through its I-BAR domain 26 or by altering the structure of the actin cytoskeleton through its interactions with cortactin or Daam1 formin 63,64 . It is also interesting to note that in contrast to earlier reports about MTSS1 to activate Rac 37 , but consistent with Rac's dispensability for I-BAR-induced filopodia formation 34 , we observed a reduction of Rac activity in HMEL cells upon MTSS1 expression. In addition to the use of different cell systems, the contextual role of extracellular cues such as fibronectin as used in our study could be a critical determinant of modulation of Rac activity by MTSS1.…”

“…MTSS1 mutants, IMDmut (K146A, K149A, K150A) and WH2mut (K746A, K747A), have been generated previously 34,35 . Transient transfections into human melanoma cells were performed using Lipofectamine (Invitrogen) according to the manufacturer's instructions.…”

“…MTSS1 senses and generates membrane curvature through its N-terminal I-BAR domain (inverse Bin/Amphiphysin/Rvs domain, also known as IRSp53/MIM homology domain or I-BAR/IMD) and interacts with actin monomers through its C-terminal WASP homology 2 (WH2) domain 26,34,35 . Furthermore, studies by the group of L. M. Machesky demonstrated that MTSS1 mediates activation of Rac via its I-BAR domain 36,37 .…”

Rigid amphipathic fusion inhibitors, small molecule antiviral compounds against enveloped viruses

“…These characteristics disfavor the formation of the negative curvature required for fusion, and the activation energy required to overcome this limitation is provided solely by the virion glycoproteins (21). In contrast, fusion of cellular vesicles is facilitated by lipidmodifying enzymes, translocases, ATPases, and other proteins that actively modulate changes in curvature (39)(40)(41)(42)(43).…”

Enterohemorrhagic Escherichia coli raises the I-BAR

“…Each interacts with phosphatidylinositol 4,5-bisphosphate in the membrane and bends the membrane via electrostatic forces (11). Expression of IMD from IRSp53 or IRTKS in cells produces filopodia-like protrusive structures of the plasma membrane, and interaction of these IMDs with phospholipidcontaining membrane vesicles in vitro induces the formation of membrane tubules (11,12). Within IMD-induced filopodia, the IMD lies just beneath the membrane, associated with the inner leaflet of the membrane and the periphery of the central actin bundle (12).…”