LAUSR

Arp2/3-dependent formation of nuclear F-actin networks of different morphology and stability is observed in an increasing number of biological processes. In CD4 T cells, T cell receptor (TCR) signaling induces cytoplasmic and nuclear F-actin assembly via Arp2/3 to strengthen contacts to antigen presenting cells and to regulate gene expression, respectively. How Arp2/3 complex is regulated to mediate these distinct actin polymerization events in response to a common stimulus is unknown. Arp2/3-complex consists of 7 subunits where ARP3, ARPC1 and ARPC5 exist as two different isoforms in humans that can assemble in complexes with different properties. Examining whether specific Arp2/3 subunit isoforms govern distinct actin remodeling events in CD4 T cells, we find that the ARPC5L isoform drives nuclear actin polymerization, while cytoplasmic actin dynamics and TCR proximal signalling selectively relies on ARPC5. In contrast, formation of stable nuclear F-actin networks triggered by DNA replication stress in CD4 T cells requires ARPC5 and is independent of ARPC5L. Moreover, nuclear actin polymerization induced by TCR signaling but not by DNA replication stress is controlled by nuclear calcium-calmodulin signalling and N-WASP. Specific ARPC5 isoforms thus govern Arp2/3 complex activity in distinct actin polymerization events. ARPC5 isoform diversity thus emerges as a mechanism to tailor Arp2/3 activity to different physiological stimuli.