Identification Of Tomosyn As A New Downstream Target Of Akt

Insulin stimulates glucose uptake into skeletal muscle and adipose tissues by increasing the available number of glucose transporter (GLUT) 4 on the cell surface. GLUT4‐loaded vesicles are targeted to plasma membrane from the intracellular reservoir through multiple trafficking steps that are regulated by protein kinase Akt. Although many clues that regulate targeting and docking of GLUT4‐vesicles to the plasma membrane had been identified, it is still largely unknown how the membrane fusion is promoted by insulin. Thus, we tried to identify a new target of Akt which is responsible for up‐regulation of SNARE‐mediated membrane fusion in insulin‐stimulated cell. We first searched for proteins bearing the recognized motif to be phosphorylated by Akt on databases. Among them, an Akt‐site was highly conserved at Ser‐783 in rat tomosyn, a SNARE binding protein, which might be involved in GLUT4 trafficking. Both Akt1 and Akt2 phosphorylated recombinant tomosyn in vitro , and the phosphorylation was diminished by replacing Ser‐783 with Ala (S783A). Protein kinase A (PKA) also phosphorylated tomosyn, but S783 is not the residue phosphorylated by PKA. The interaction of tomosyn and syntaxin 4 was modulated by the phosphorylation. The results suggest that tomosyn would be involved in insulin‐triggered membrane fusion between docked GLUT4 vesicle and the target plasma membrane.