Molecular principles of bidirectional signalling between membranes and small GTPases

Abstract

Most small GTPases actuate their functions on subcellular membranes, which are increasingly seen as integral components of small GTPase signalling. In this review, we used the highly studied regulation of Arf GTPases by their GEFs to categorize the molecular principles of membrane contributions to small GTPase signalling, which have been highlighted by integrated structural biology combining in vitro reconstitutions in artificial membranes and high‐resolution structures. As an illustration of how this framework can be harnessed to better understand the cooperation between small GTPases, their regulators and membranes, we applied it to the activation of the small GTPase Rac1 by DOCK‐ELMO, identifying novel contributions of membranes to Rac1 activation. We propose that these structure‐based principles should be considered when interrogating the mechanisms whereby small GTPase systems ensure spatial and temporal control of cellular signalling on membranes.

CED-5/CED-12 (DOCK/ELMO) can promote and inhibit F-actin formation via distinct motifs that may target different GTPases

“…We do not favor this interpretation. First, DOCK GEFs have never been shown to activate RHO-1 [ 53 – 55 ]. Second, the structure for DOCK/ELMO/Rac1 argues against this.…”

RhoG facilitates a conformational transition in the guanine nucleotide exchange factor complex DOCK5/ELMO1 to an open state

“…ELMO1 NTD appeared disordered in the DOCK5/ELMO1/Rac1 complex, whereas in the DOCK2/ELMO1/Rac1 complex, it was structured into an S-shaped open conformation ( 21 ). In the S-shaped conformation of ELMO1 NTD , the symmetric DOCK2/ELMO1 dimer and RhoG cannot simultaneously interact with the membrane ( 25 ). Therefore, it remains unclear how the DOCK/ELMO complex is activated by membrane-localized RhoG.…”

Small GTPase signaling hubs at the surface of cellular membranes in physiology and disease

“…Arf GTPases, which are major regulators of most aspects of cellular traffic, form a more remote subfamily (to which Sar GTPases and possibly the Rag GTPases and bacterial mutual gliding‐motility protein A [MglA] are related), characterized by a unique allosteric mechanism whereby the nucleotide‐binding site communicates with the membrane [14]. Nawrotek, Cherfils, and co‐workers present an overview of the various ways in which Arfs and their GEFs exchange bidirectional information with the membrane [9]. Accordingly, they define a framework to analyze the interactions of other regulators and effectors of small GTPases with membranes, exemplified by a structure‐based perspective of the activation of DOCK GEFs on membranes.…”

“…We are now delighted to present 13 articles that highlight how integrated structural, biochemical, and biophysical approaches have allowed to shed light on the mechanisms of fundamental functions of major members of this superfamily. Notably, these reviews show how several experimental approaches that were rarely used a decade ago, are now taking center stage in moving the field forward: reconstitutions of small GTPase systems in artificial membranes [5][6][7][8][9], single particle cryo-electron microscopy [5][6][7][9][10][11][12], 3D electron tomography reconstructions [12], or optogenetics [13]. Of these, Loose and coauthors present a great overview of the power and inherent difficulties of in vitro reconstitutions using biomimetic membranes, and how they have revealed in a quantitative manner the inner workings of intricate small GTPase networks, which would have been unattainable in vivo [8].…”