RNA binding protein CAPRIN1 suppresses STAT1 translation and interferon signalling to promote HBV replication.

BACKGROUND A G-quadruplex (rG4) structure within 5' untranslated region (5'UTR) of Signal Transducer and Activator of Transcription 1 (STAT1) messenger RNA (mRNA) functions as a translational brake modulating STAT1 translation and regulating interferon (IFN) response. We hypothesised that rG4-interacting proteins could be pivotal modulators of STAT1 expression and, consequently, IFN therapy for HBV resistance. OBJECTIVE The study aims to determine the role of the STAT1 rG4 binding protein candidate CARPIN1 in IFN signalling and to elucidate how HBV infection drives CARPIN1 expression. DESIGN AND RESULTS Employing an integrated platform, complemented by HBV-infection models, humanised liver mice and paired liver biopsies, we found that cell cycle associated protein 1 (CAPRIN1) facilitates stress granule formation and stabilises STAT1 rG4, thereby repressing STAT1 translation. Quantitative assays confirm an inverse relationship: IFN non-responders exhibit high CAPRIN1 and low STAT1, whereas IFN responders display the opposite profile. Mechanistically, HBV polymerase functions as a transcription factor that drives CAPRIN1 expression. CAPRIN1 knockdown in vitro and in vivo restores STAT1 abundance and sensitises cells to IFN, whereas re-expression establishes IFN resistance. Ribonucleoprotein immunoprecipitation-Mass spectrometry, electrophoretic mobility shift assay, luciferase reporter assays, ribosome profiling and circular dichroism analyses collectively demonstrate that CAPRIN1 selectively binds STAT1 rG4, halting ribosomal scanning and suppressing STAT1 protein production. IFN-resistant cells mirror these findings, displaying elevated CAPRIN1 and diminished STAT1. CONCLUSION CAPRIN1 is elevated in IFN non-responders and further upregulated during HBV infection/replication. By facilitating stress granule formation and stabilising STAT1 rG4 structure, CAPRIN1 blocks ribosomal scanning and suppresses STAT1 translation. We therefore designate CAPRIN1 as a critical rheostat that calibrates the amplitude of IFN responses during innate immunity and adjuvant IFN-α therapy.