Insulin causes the rapid translocation of the glucose transporter GLUT4 from intracellular sites to the plasma membrane in fat and muscle cells. There is considerable evidence that the signaling to this traflicking process is downstream of the insulin-activated protein kinase Akt. One Akt substrate that connects signaling to traflicking is a 160 Wit GTPase activating protein for Rabs. Another potential connecting substrate is the protein Synip, which associates with the SNARE syntaxin4, A recent study presents evidence that Akt phosphorylates Synip on serine 99, at least in vitro, and proposes that this phosphorylation enables GLUT4 translocation by causing the dissociation of Synip from syntaxin4. In the present Study we show that marked overexpression of Synip mutant S99A, which lacks this phosphorylation site, has no effect on insulin-stimulated GLUT4 translocation in 3T3-L1 adipocytes. This finding is strong evidence that phosphorylation of Synip on serine 99 is not required for GLUT4 translocation. (c) 2005 Elsevier Inc. All rights reserved.