Cryo-EM of human Arp2/3 complexes provides structural insights into actin nucleation modulation by ARPC5 isoforms.

Ottilie von Loeffelholz,Carolyn A Moores,Naoko Kogata,Svend Kjaer,Andrew Purkiss,Luyan Cao,Michael Way,Guillaume Romet-Lemonne

doi:10.1242/bio.054304

Abstract

ABSTRACTThe Arp2/3 complex regulates many cellular processes by stimulating formation of branched actin filament networks. Because three of its seven subunits exist as two different isoforms, mammals produce a family of Arp2/3 complexes with different properties that may be suited to different physiological contexts. To shed light on how isoform diversification affects Arp2/3 function, we determined a 4.2 Å resolution cryo-EM structure of the most active human Arp2/3 complex containing ARPC1B and ARPC5L, and compared it with the structure of the least active ARPC1A-ARPC5-containing complex. The architecture of each isoform-specific Arp2/3 complex is the same. Strikingly, however, the N-terminal half of ARPC5L is partially disordered compared to ARPC5, suggesting that this region of ARPC5/ARPC5L is an important determinant of complex activity. Confirming this idea, the nucleation activity of Arp2/3 complexes containing hybrid ARPC5/ARPC5L subunits is higher when the ARPC5L N-terminus is present, thereby providing insight into activity differences between the different Arp2/3 complexes.

Highlights

The Arp2/3 complex is evolutionarily conserved and built from two actin related proteins (Arp2 and Arp3) and five other protein subunits (ARPC1-5) (Molinie and Gautreau, 2018, Goley and Welch, 2006)
We show how different Arp2/3 subunit isoforms contribute to differences in complex function
Because three of its seven subunits exist as two different isoforms, mammals produce a family of Arp2/3 complexes with different properties that may be suited to different physiological contexts

Summary

Introduction

The Arp2/3 complex is evolutionarily conserved and built from two actin related proteins (Arp and Arp3) and five other protein subunits (ARPC1-5) (Molinie and Gautreau, 2018, Goley and Welch, 2006). In mammals, Arp, ARPC1 and ARPC5 are present as two isoforms - Arp, Arp3B, ARPC1A, ARPC1B, ARPC5 and ARPC5L - that share 91, 67 and 67% sequence identity respectively (Abella et al, 2016, Pizarro-Cerda et al, 2017). This raises the question as to whether different Arp2/3 complexes have evolved unique properties that are adapted to their particular cellular, developmental or physiological roles. Tissue-specific expression patterns of subunit isoforms, together with isoformspecific susceptibility to disease-causing point mutations, point to distinct physiological roles for particular Arp2/3 isoforms in nuclei positioning in skeletal muscle (Roman et al, 2017), as well as cytotoxic T lymphocyte maintenance and activity (Brigida et al, 2018, Kuijpers et al, 2017, Randzavola et al, 2019, Somech et al, 2017, Volpi et al, 2019, Roman et al, 2017, Kahr et al, 2017)

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