biostudies-literatureKeplinger AJCancer Research FoundationNational Institutes of HealthNational Institute of General Medical SciencesNational Science Foundation111123https://www.ebi.ac.uk/biostudies/studies/S-EPMC12860355biostudies-literatureUnknown302(2)While predominantly studied in cytoplasmic contexts, actin plays critical roles in the nucleus, regulating genome accessibility, transcription, and DNA repair. Cell-based studies have contributed to a widely accepted model in which the import factor importin 9 (IPO9) acts in concert with the actin filament-severing protein cofilin to transport actin into the nucleus. The classical nuclear localization signal on cofilin is thought to anchor IPO9 to cofilin-bound actin monomers, driving the formation of an import-competent tripartite actin-cofilin-IPO9 complex. Contrary to this established model of actin import, we demonstrate that IPO9 directly binds to monomeric actin with midnanomolar affinity and, rather than promoting IPO9-actin complex formation, cofilin competitively inhibits the bindThe Journal of biological chemistryA revised model of nuclear actin import: Importin 9 competes with cofilin, profilin, and RanGTP for actin binding.PMC12860355R35-GM145373Ruthenburg AJSrinivasan PAKeplinger AJChristensen SMSuarez CfalseA revised model of nuclear actin import: Importin 9 competes with cofilin, profilin, and RanGTP for actin binding.While predominantly studied in cytoplasmic contexts, actin plays critical roles in the nucleus, regulating genome accessibility, transcription, and DNA repair. Cell-based studies have contributed to a widely accepted model in which the import factor importin 9 (IPO9) acts in concert with the actin filament-severing protein cofilin to transport actin into the nucleus. The classical nuclear localization signal on cofilin is thought to anchor IPO9 to cofilin-bound actin monomers, driving the formation of an import-competent tripartite actin-cofilin-IPO9 complex. Contrary to this established model of actin import, we demonstrate that IPO9 directly binds to monomeric actin with midnanomolar affinity and, rather than promoting IPO9-actin complex formation, cofilin competitively inhibits the bind2025-01-01T00:00:00Z2025 Dec2026-06-12T10:15:29.187Z2026-06-12T03:11:40.774ZS-EPMC128603554147857010.1016/j.jbc.2025.111123