Francis Justus, Pathri Achyutha Krishna, Shyam Kankipati Teja, Sripada Sridhar, Mitra Rishav, Narvaez-Ortiz Heidy Y, Eliyan Kiran Vyshnav, Nolen Brad J, Chowdhury Saikat
CSIR-Centre for Cellular and Molecular Biology, Hyderabad, India.
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India.
Nat Struct Mol Biol. 2025 Sep 15. doi: 10.1038/s41594-025-01673-8.
Arp2/3 complex is a key nucleator of actin filaments. It requires activation by nucleation-promoting factors (NPFs). WISH/DIP1/SPIN90 (WDS) proteins represent a unique class of NPFs that activate the Arp2/3 complex independently of preexisting filaments, promoting linear actin-filament nucleation. In fission yeast, Dip1 binds to the clamp subunits in Arp2/3 complex to induce the short-pitch conformation, where Arp2 moves closer to Arp3 to mimic a filamentous actin dimer. However, how WDS proteins stimulate subunit flattening in Arp subunits, a 'scissor-like' conformational change akin to what is observed in an actin monomer during filament formation, remained unclear. Here we present cryo-electron microscopy structures of human SPIN90 bound to activated bovine Arp2/3 complex on an actin filament pointed end. The structures show that SPIN90 dimerizes through a metazoan-specific domain in the middle segment, engaging both the clamp and the Arp3/ARPC3 interface, to drive the activating conformational changes in Arp2/3 complex. Remarkably, a single SPIN90 dimer can also bridge two Arp2/3 complexes, enabling bidirectional actin nucleation and suggesting a mechanism for rapidly assembling complex actin network architectures.
Arp2/3复合物是肌动蛋白丝的关键成核因子。它需要成核促进因子（NPFs）激活。WISH/DIP1/SPIN90（WDS）蛋白代表一类独特的NPFs，可独立于预先存在的丝激活Arp2/3复合物，促进线性肌动蛋白丝成核。在裂殖酵母中，Dip1与Arp2/3复合物中的钳夹亚基结合以诱导短间距构象，其中Arp2向Arp3靠近以模拟丝状肌动蛋白二聚体。然而，WDS蛋白如何刺激Arp亚基中的亚基变平，即类似于在丝形成过程中肌动蛋白单体中观察到的“剪刀状”构象变化，仍不清楚。在此，我们展示了人SPIN90与肌动蛋白丝尖端的活化牛Arp2/3复合物结合的冷冻电镜结构。这些结构表明，SPIN90通过中间段的后生动物特异性结构域二聚化，与钳夹和Arp3/ARPC3界面结合，以驱动Arp2/3复合物的活化构象变化。值得注意的是，单个SPIN90二聚体还可以桥接两个Arp2/3复合物，实现双向肌动蛋白成核，并提示了一种快速组装复杂肌动蛋白网络结构的机制。
