Phosphoinositides containing stearic acid are required for interaction between Rho GTPases and the exocyst to control the late steps of polarised exocytosis

Cell polarity is achieved by regulators such as small G proteins, exocyst members and phosphoinositides, with the latter playing a key role when bound to the exocyst proteins Sec3p and Exo70p, and Rho GTPases. This ensures asymmetric growth via the routing of proteins and lipids to the cell surface using actin cables. Previously, using a yeast mutant for a lysophosphatidylinositol acyl transferase encoded by the PSI1 gene, we demonstrated the role of stearic acid in the acyl chain of phosphoinositides in cytoskeletal organisation and secretion. Here, we use a genetic approach to characterise the effect on late steps of the secretory pathway. The constitutive overexpression of PSI1 in mutants affecting kinases involved in the phosphoinositide pathway demonstrated the role of molecular species containing stearic acid in bypassing a lack of phosphatidylinositol-4-phosphate PI(4)P at the plasma membrane, which is essential for the function of the Cdc42p module. Decreasing the levels of stearic acid-containing phosphoinositides modifies the environment of the actors involved in the control of late steps in the secretory pathway. This leads to decreased interactions between Exo70p and Sec3p, with Cdc42p, Rho1p and Rho3p, due to disruption of the GTP/GDP ratio of at least Rho1p and Rho3p GTPases, thereby preventing activation of the exocyst.

The phosphoinositide code is read by a plethora of protein domains

INTRODUCTION

The proteins that decipher nucleic acid- and protein-based information are well known, however, those that read membrane-encoded information remain understudied. Here we report 70 different human, microbial and viral protein folds that recognize phosphoinositides (PIs), comprising the readers of a vast membrane code.

AREAS COVERED

Membrane recognition is best understood for FYVE, PH and PX domains, which exemplify hundreds of PI code readers. Comparable lipid interaction mechanisms may be mediated by kinases, adjacent C1 and C2 domains, trafficking arrestin, GAT and VHS modules, membrane-perturbing annexin, BAR, CHMP, ENTH, HEAT, syntaxin and Tubby helical bundles, multipurpose FERM, EH, MATH, PHD, PDZ, PROPPIN, PTB and SH2 domains, as well as systems that regulate receptors, GTPases and actin filaments, transfer lipids and assembled bacterial and viral particles.

EXPERT OPINION

The elucidation of how membranes are recognized has extended the genetic code to the PI code. Novel discoveries include PIP-stop and MET-stop residues to which phosphates and metabolites are attached to block phosphatidylinositol phosphate (PIP) recognition, memteins as functional membrane protein apparatuses, and lipidons as lipid "codons" recognized by membrane readers. At least 5% of the human proteome senses such membrane signals and allows eukaryotic organelles and pathogens to operate and replicate.

Role of RhoGAP Rgd1 in Pkc1 signaling-related actin repolarization under heat shock stress in Saccharomyces cerevisiae

BACKGROUND

A serine/threonine kinase Pkc1 is the sole protein kinase C in the budding yeast Saccharomyces cerevisiae, and plays an important role in the regulation of polarized growth and stress responses such as those due to heat shock. Exposure of cells to high temperature transiently arrests polarized growth and leads to depolarization of the actin cytoskeleton, followed by actin repolarization during adaptation to heat shock stress. Actin repolarization is ensured by the activation of Pkc1 signaling; however, the molecular mechanisms underlying this phenomenon remain poorly understood.

METHODS

Using an overexpression construct of a constitutively active mutant of Pkc1 (Pkc1^R398P), we explored the Pkc1 target molecules involved in actin repolarization.

RESULTS

PKC1^R398P overexpression as well as heat shock stress increased the phosphorylation levels of Rho GTPase-activating protein (RhoGAP) Rgd1. Rgd1 was found to contribute to Pkc1-signaling-related actin repolarization during adaptation to heat shock stress in a GAP activity-independent manner, with Ser148 in Rgd1 playing a crucial role. Furthermore, Rgd1 was involved in the maintenance of phosphorylation status of the mitogen-activated protein (MAP) kinase Mpk1, a downstream effector of Pkc1, under heat shock stress.

CONCLUSIONS

Rgd1 is a target of Pkc1 signaling under conditions of heat shock stress, and required for the normal process of actin repolarization during adaptation to heat shock stress.

GENERAL SIGNIFICANCE

Our results provide insights into the molecular mechanism underlying Pkc1-mediated modulation of actin repolarization under heat shock stress.

Assignment of 1H, 13C and 15N resonances and secondary structure of the Rgd1-RhoGAP domain

The protein Rgd1 is involved in the regulation of cytoskeleton formation and in signalling pathways that control cell polarity and growth in Saccharomyces cerevisiae. Rgd1p is composed of a F-BAR domain required for membrane binding and a RhoGAP domain responsible for activating Rho3p and Rho4p, two GTPases respectively involved in bud growth and cytokinesis. Rgd1p is recruited to the membrane through interactions with phosphoinositide lipids, which bind the two isolated domains and stimulate the RhoGAP activity on Rho4p. As previously shown by crystallography, the membrane-binding F-BAR domain contains a conserved inositol phosphate binding site, which explains the preferential binding of phosphoinositides. In contrast, RhoGAP domains are not expected to bind lipids. In order to unravel this puzzling feature, we solved the three-dimensional structure of the isolated protein and found a cryptic phosphoinositide binding site involving non conserved residues (Martinez et al. 2017). The assignment of the resonances and secondary structure of Rgd1-RhoGAP (aa 450-666) is presented here.