Ryu, J. E.; Park, M.; Roh, H. W.; Lee, J.-H.; Kim, E. Y.

Astrocytes contain cell-autonomous circadian clocks, but how astrocyte-enriched clock-controlled genes feed back onto the circadian clock remains poorly understood. Here, we identify transmembrane protein 44 (Tmem44) as an astrocyte-enriched circadian transcript whose protein product regulates clock protein abundance through translational control. Tmem44 mRNA oscillated in cultured mouse cortical astrocytes in a BMAL1-dependent manner, whereas TMEM44 protein was constitutively expressed and localized to the endoplasmic reticulum. Acute Tmem44 knockdown reduced BMAL1 and PER2 protein levels without altering their mRNA levels or degradation kinetics, and dampened Per2-luciferase circadian rhythm amplitude. SUnSET assays revealed that Tmem44 knockdown decreased global nascent protein synthesis. Proximity labeling and co-immunoprecipitation further showed that TMEM44 associates with ribosomal proteins and ER-associated translational machinery. Importantly, global protein synthesis exhibited circadian oscillation in synchronized astrocytes, and this rhythmic translation was abolished by Tmem44 knockdown. These findings identify TMEM44 as an astrocyte-enriched, ER-resident translational regulator that supports rhythmic protein synthesis and sustains high-amplitude clock protein oscillations in astrocytes.