CRY1 Lysine 151 Regulates Circadian Rhythms Through Ubiquitination-Independent Protein Interactions

Mammalian circadian rhythms, governing ~24 h oscillations in behavior, physiology, and hormone levels, are orchestrated by transcriptional-translational feedback loops centered around the core clock protein cryptochrome 1 (CRY1). While CRY1 ubiquitination is known to regulate clock function, the roles of specific ubiquitination sites remain unclear. Here, we identify lysine 151 (K151) as a critical residue modulating the circadian period through non-canonical mechanisms. Using site-directed mutagenesis, we generated CRY1-K151Q/R mutants mimicking constitutive deubiquitination. Circadian rescue assays in Cry1/2-deficient cells revealed period shortening (K151Q: -2.25 h; K151R: -1.4 h; n = 3, <i>p</i> < 0.01, Student's <i>t</i>-test), demonstrating K151's functional importance. Despite normal nuclear localization kinetics, K151Q/R mutants exhibited reduced transcriptional repression in luciferase assays, a weakened interaction with BMAL1 by the luciferase complementation assay, and enhanced binding to E3 ligase FBXL12 (but not FBXL3) while showing more stability than wild-type CRY1. Notably, the absence of ubiquitination-linked degradation or altered FBXL3 engagement suggests a ubiquitination-independent mechanism. We propose that CRY1-K151 serves as a structural hub fine-tuning circadian periodicity by modulating core clock protein interactions rather than through traditional ubiquitin-mediated turnover. These findings redefine the mechanistic landscape of post-translational clock regulation and offer new therapeutic avenues for circadian disorders.