Mapping of the contraction-induced phosphoproteome identifies TRIM28 as a significant regulator of skeletal muscle size and function

Mechanical signals, such as those evoked by maximal-intensity contractions (MICs), can induce an increase in muscle mass. Rapamycin-sensitive signaling events are widely implicated in the regulation of this process; however, recent studies indicate that rapamycin-insensitive signaling events are also involved. Thus, to identify these events, we generate a map of the MIC-regulated and rapamycin-sensitive phosphoproteome. In total, we quantify more than 10,000 unique phosphorylation sites and find that more than 2,000 of these sites are significantly affected by MICs, but remarkably, only 38 of the MIC-regulated events are mediated through a rapamycin-sensitive mechanism. Further interrogation of the rapamycin-insensitive phosphorylation events identifies the S473 residue on Tripartite Motif-Containing 28 (TRIM28) as one of the most robust MIC-regulated phosphorylation sites, and extensive follow-up studies suggest that TRIM28 significantly contributes to the homeostatic regulation of muscle size and function as well as the hypertrophy that occurs in response to increased mechanical loading. [Display omitted] •Extensive map of the rapamycin-sensitive and contraction-regulated phosphoproteome•Contractions induce robust, rapamycin-insensitive phosphorylation of TRIM28(S473)•TRIM28(S473) phosphorylation is sufficient to increase skeletal myofiber size•Myofiber KO of TRIM28 alters muscle size, function, and mechanical load-induced hypertrophy By comprehensively mapping the rapamycin-sensitive and contraction regulated phosphoproteome, Steinert et al. identify TRIM28 as a non-canonical regulator of muscle size and function. This study provides a resource for further delineating the signaling components through which mechanical stimuli regulate mTOR and muscle growth.