Control of Formin Distribution and Actin Cable Assembly by the E3 Ubiquitin Ligases Dma1 and Dma2

Abstract

Formins are widespread actin-polymerizing proteins that play pivotal roles in a number of processes, such as cell polarity, morphogenesis, cytokinesis, and cell migration. In agreement with their crucial function, formins are prone to a variety of regulatory mechanisms that include autoinhibition, post-translational modifications, and interaction with formin modulators. Furthermore, activation and function of formins is intimately linked to their ability to interact with membranes. In the budding yeast Saccharomyces cerevisiae, the two formins Bni1 and Bnr1 play both separate and overlapping functions in the organization of the actin cytoskeleton. In addition, they are controlled by both common and different regulatory mechanisms. Here we show that proper localization of both formins requires the redundant E3 ubiquitin ligases Dma1 and Dma2, which were previously involved in spindle positioning and septin organization. In dma1 dma2 double mutants, formin distribution at polarity sites is impaired, thus causing defects in the organization of the actin cable network and hypersensitivity to the actin depolymerizer latrunculin B. Expression of a hyperactive variant of Bni1 (Bni1-V360D) rescues these defects and partially restores proper spindle positioning in the mutant, suggesting that the failure of dma1 dma2 mutant cells to position the spindle is partly due to faulty formin activity. Strikingly, Dma1/2 interact physically with both formins, while their ubiquitin-ligase activity is required for formin function and polarized localization. Thus, ubiquitylation of formin or a formin interactor(s) could promote formin binding to membrane and its ability to nucleate actin. Altogether, our data highlight a novel level of formin regulation that further expands our knowledge of the complex and multilayered controls of these key cytoskeleton organizers. KEYWORDS actin; formin; ubiquitylation; budding yeast T HE ability to polarize is a fundamental property of all types of cells, being crucial for numerous cellular processes such as proliferation, differentiation, and morphogenesis. Indeed, dysregulation of cell polarity can underlie developmental disorders and cancers (Wodarz and Nathke 2007). Cell polarization is strictly linked to the reorganization of the cytoskeleton and in particular of the actin network, whose dynamics must be tightly controlled for polarized processes to occur properly.The unicellular budding yeast Saccharomyces cerevisiae divides asymmetrically and undergoes highly polarized cell growth throughout its life cycle. Most aspects of polarized growth in budding yeast arise from a precise arrangement of the cortical actin cytoskeleton during the cell cycle. Three main actin structures can be found in yeast cells: (i) actin patches, which are sites of active endocytosis, (ii) actin cables, which serve as tracks for polarized secretion and segregation of organelles, and (iii) the contractile acto-myosin ring, which is involved in cytokinesis (Adams and Pringle 1984;Kilmartin and Adams 1984;Bi et ...

Targeted substrate degradation by Kelch controls the actin cytoskeleton during ring canal expansion

“…Regulation of cytoskeletal structures by the ubiquitin-proteasome system is an emerging regulatory mechanism (Deng and Huang, 2014). However, most previous studies have used yeast or cultured cells (Wang et al, 2003;Chen et al, 2009;Razinia et al, 2011Razinia et al, , 2013Juanes and Piatti, 2016;Girouard et al, 2016;Wu et al, 2018). Our work is one of few examples to date detailing the mechanism by which the UPS organizes and remodels a cytoskeletal structure in a metazoan developmental model system.…”

A pair of E3 ubiquitin ligases compete to regulate filopodial dynamics and axon guidance

“…Further studies need to investigate the number and arrangement of ubiquitin moieties on VASP and how ubiquitination of VASP alters its function on the molecular level or changes the state of the tetramer. Such work could have intriguing implications for the regulation of other actin-associated proteins that function as multimers, such as formins, which recent work has suggested are also regulated by nondegradative ubiquitination (Juanes and Piatti, 2016).…”

A fungal arrestin protein contributes to cell cycle progression and pathogenesis

“…We also observed that Ali1 interacts with multiple ubiquitination proteins, including E1, E2, and E3 proteins, particularly in TC conditions (73). Ubiquitination is most often considered in the context of proteasomal degradation, but it can also direct diverse subcellular localizations (74–76).…”