SANS ankyrin repeats mediate non-classical nuclear import and dictate distinct pathogenic mechanisms

Mutations in the USH1G gene, which encodes the scaffold protein SANS, cause type-I Usher syndrome, a severe inherited disorder involving both hearing and vision loss. Although SANS has been implicated in nuclear pre-mRNA splicing, the molecular determinants of its nuclear import and their relevance to USH1G pathogenesis remain incompletely understood. Here, we report high-resolution crystal structures of the N-terminal ankyrin repeat regions of SANS and its homolog ANKS4B. We show that SANS_AR functions as a self-sufficient module that promotes nuclear localization, whereas ANKS4B_AR lacks this property. Structure-guided analysis of USH1G-associated variants suggests two distinct classes of effects: core hydrophobic mutations such as L48P, L84P impair SANS_AR solubility and are predicted to destabilize the domain, whereas surface-exposed substitutions such as R10W, S119P, and R146C remain compatible with soluble expression but reduce SANS nuclear localization. Together, these findings provide a molecular framework for understanding how impaired SANS_AR stability and defective nuclear localization may contribute to USH1G pathogenesis.