The Cdc42 GTPase – a central organizer of cell polarization
Small GTPases of the Rho family play essential roles in eukaryotic cell polarization. These signaling molecules act as switches, being active in GTP-bound and inactive in GDP-bound forms. In yeast, the Rho-family GTPase Cdc42 is a key regulator. Cdc42 is active at cell poles where is promotes polarized cell growth. How the cell activates Cdc42 specifically at cell poles, how the size of the Cdc42-active zone is defined, and how Cdc42 then promotes polarized growth are some of the questions we are aiming to address.
A functionally-tagged Cdc42 allele, in which a fluorescent protein, mCherry or sfGFP, is inserted in a non-conserved internal site shows Cdc42 localizes to most cellular membranes (top). In contrast, its active form (middle) is present exclusively at the poles of the cell, where it promotes polarized growth. Our work aims to understand how Cdc42 activity is concentrated and restricted at these locations.
Bendezú FO, Vincenzetti V, Vavylonis D, Wyss R, Vogel H, Martin SG. Spontaneous Cdc42 polarization independent of GDI-mediated extraction and actin-based trafficking. PLoS Biol. 2015 Apr 2;13(4):e1002097. doi:10.1371/journal.pbio.1002097. PMID: 25837586.
Gallo Castro D, Martin SG. Differential GAP requirement for Cdc42-GTP polarization during proliferation and sexual reproduction. J Cell Biol. 2018 Dec 3;217(12):4215-4229. doi: 10.1083/jcb.201806016. PMID: 30279276.
Organization of actin cables for polarized cell growth
Vegetative yeast cells exhibit three distinct actin structures: the cytokinetic actin ring necessary for cell division and actin cables that underlie polarized vesicle transport, assembled by formin nucleators, and actin patches at sites of endocytosis, assembled by Arp2/3. Actin cables are assembled by the formin For3 and serve as tracks for myosin V-dependent transport of cargoes towards sites of polarized cell growth. Cargoes include membrane material and cell wall remodeling components essential for polarized cell growth. Our work has focused on the regulation of the formin For3, which is controlled by Cdc42, as well as a second formin Fus1 required during sexual reproduction (see here). For instance, we previously showed that both actin cables and For3 undergo retrograde flow from the cell poles and that type V myosins contribute to the organization of actin cables.
Role of the exocyst for polarized cell growth
Myosin V transport and actin cables, while important for polarized cell growth, are not essential for it. Indeed, fission yeast cells also use a second Cdc42-dependent polarization strategy. Secretion of cell wall-remodelling components at cell poles requires the exocyst complex, an eight-subunit complex that facilitates fusion of vesicles with the plasma membrane. Localization of this complex to cell poles is independent of the cytoskeleton. Disruption of actin cables or of the exocyst does not block polarized growth, but double disruption prevents polarized growth. For polar growth at cell poles, fission yeast thus rely both on the actin cables that transport vesicle cargoes and on the exocyst complex that promotes fusion of these vesicles with the plasma membrane.