Characterization of Sin1 Isoforms Reveals an mTOR-Dependent and Independent Function of Sin1γ

Sin1 or MAPKAP1 is a key component of mTORC2 signaling complex which is necessary for AKT phosphorylation at the S473 and T450 sites, and also for AKT downstream signaling as well. A number of Sin1 splicing variants have been reported that can produce different Sin1 isoforms due to exon skipping or alternative transcription initiation. In this report, we characterized four Sin1 isoforms, including a novel Sin1 isoform due to alternative 3' termination of the exon 9a, termed Sin1 gamma. Sin1 gamma expression can be detected in multiple adult mouse tissues, and it encodes a C-terminal truncated protein comparing to the full length Sin1 beta isoform. In contrast to Sin1 beta, Sin1 gamma overexpression in Sin1 deficient mouse embryonic fibroblasts has no significant impact on mTORC2 activity or mTORC2 subunits protein level, although it still can interact with mTORC2 components. More interestingly, Sin1 gamma was detected in a specific cytosolic location with a distinct feature in structure, and its localization was transiently disrupted during cell cycle. Therefore, Sin1 gamma is a novel Sin1 isoform and may have distinct properties in cell signaling and intracellular localization from other Sin1 isoforms.