MTSS1-dependent ubiquitin modifications mediated by FBXO44 remodel the actin cytoskeleton to promote gastric cancer progression.

Dynamic cytoskeletal homeostasis drives malignant transformation in tumor cells and represents a therapeutic vulnerability. Therapeutic targeting of this equilibrium may improve outcomes for cancer patients. Rac1 acts as a central molecular switch that controls actin cytoskeleton dynamics. Although multiple biological strategies modulate its spatiotemporal activity to maintain actin cytoskeleton homeostasis, the underlying molecular mechanisms remain unclear. Here, we identify FBXO44 as a critical regulator of Rac1 nucleocytoplasmic trafficking via its interaction with MTSS1. In gastric cancer (GC), FBXO44 directs two distinct ubiquitination programs on MTSS1: K63-linked polyubiquitination of MTSS1 promotes Rac1 nuclear translocation, whereas K11-linked polyubiquitination induces proteasomal degradation of MTSS1, restricting Rac1 nuclear entry. This ubiquitin-mediated coordination reprograms nucleocytoplasmic Rac1 signaling distribution and remodels the actin cytoskeleton. Structural analysis demonstrates that FBXO44 binds MTSS1 via distinct domains, dynamically balancing these opposing ubiquitination events by controlling MTSS1 abundance, thereby fine-tuning actin cytoskeletal dynamics. Clinically, this regulatory axis supports an oncogenic phenotype: FBXO44 overexpression correlates with enhanced Rac1 signaling and activation of associated pathways in advanced GC. Importantly, the expression balance of the FBXO44/MTSS1 axis significantly influences patient prognosis. Our findings provide mechanistic insights into cytoskeletal regulation and establish a translational framework for GC therapy.