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Published online 7 March 2005. doi:10.1083/jcb.200408182
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 168, Number 6, 921-928
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Article

Akt2 phosphorylates Synip to regulate docking and fusion of GLUT4-containing vesicles



Eijiro Yamada1, Shuichi Okada1, Tsugumichi Saito1, Kihachi Ohshima2, Minoru Sato1, Takafumi Tsuchiya1, Yutaka Uehara1, Hiroyuki Shimizu1, and Masatomo Mori1,3

1 Department of Medicine and Molecular Science, Gunma University Graduate School of Medicine, Maebashi, Gunma, 371-8511, Japan
2 Health and Science Center, Gunma University, Maebashi, Gunma, 371-8510, Japan
3 Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan

Correspondence to Shuichi Okada: okadash{at}showa.gunma-u.ac.jp

We have identified an unusual potential dual Akt/protein kinase B consensus phosphorylation motif in the protein Synip (RxKxRS97xS99). Surprisingly, serine 97 is not appreciably phosphorylated, whereas serine 99 is only a specific substrate for Akt2 but not Akt1 or Akt3. Although wild-type Synip (WT-Synip) undergoes an insulin-stimulated dissociation from Syntaxin4, the Synip serine 99 to phenylalanine mutant (S99F-Synip) is resistant to Akt2 phosphorylation and fails to display insulin-stimulated Syntaxin4 dissociation. Furthermore, overexpression of WT-Synip in 3T3L1 adipocytes had no effect on insulin-stimulated recruitment of glucose transporter 4 (GLUT4) to the plasma membrane, whereas overexpression of S99F-Synip functioned in a dominant-interfering manner by preventing insulin-stimulated GLUT4 recruitment and plasma membrane fusion. These data demonstrate that insulin activation of Akt2 specifically regulates the docking/fusion step of GLUT4-containing vesicles at the plasma membrane through the regulation of Synip phosphorylation and Synip–Syntaxin4 interaction.

Abbreviations used in this paper: GLUT4, glucose transporter 4; GSK3, glycogen synthesis kinase 3; PI3, phosphatidylinositol 3; siRNA, small interfering RNA; WT, wild-type.


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