Cellular Control of Cortical Actin Nucleation

Summary

The contractile actin cortex is a thin layer of actin, myosin, and actin-binding proteins that subtends the membrane of animal cells. The cortex is the main determinant of cell shape and plays a fundamental role in cell division [ 1–3], migration [ 4], and tissue morphogenesis [ 5]. For example, cortex contractility plays a crucial role in amoeboid migration of metastatic cells [ 6] and during division, where its misregulation can lead to aneuploidy [ 7]. Despite its importance, our knowledge of the cortex is poor, and even the proteins nucleating it remain unknown, though a number of candidates have been proposed based on indirect evidence [ 8–15]. Here, we used two independent approaches to identify cortical actin nucleators: a proteomic analysis using cortex-rich isolated blebs, and a localization/small hairpin RNA (shRNA) screen searching for phenotypes with a weakened cortex or altered contractility. This unbiased study revealed that two proteins generated the majority of cortical actin: the formin mDia1 and the Arp2/3 complex. Each nucleator contributed a similar amount of F-actin to the cortex but had very different accumulation kinetics. Electron microscopy examination revealed that each nucleator affected cortical network architecture differently. mDia1 depletion led to failure in division, but Arp2/3 depletion did not. Interestingly, despite not affecting division on its own, Arp2/3 inhibition potentiated the effect of mDia1 depletion. Our findings indicate that the bulk of the actin cortex is nucleated by mDia1 and Arp2/3 and suggest a mechanism for rapid fine-tuning of cortex structure and mechanics by adjusting the relative contribution of each nucleator.

Highlights

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  The Arp2/3 complex and the formin mDia1 nucleate the bulk of cortical F-actin

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  Nucleator depletion alters the cortex ultrastructural organization

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  Actin nucleated by mDia1 accumulates faster than actin nucleated by Arp2/3

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  mDia1 depletion leads to mitosis failure, an effect potentiated by Arp2/3 inhibition

Abstract

Bovellan et al. show that two actin nucleators, the formin mDia1 and the Arp2/3 complex, generate the bulk of the submembranous cortical F-actin. These nucleators have very different actin accumulation kinetics and play distinct roles in cortical actin organization, and their depletion differentially affects progression through the cell cycle.