Phosphorylation and degradation of tomosyn-2 de-represses insulin secretion

J Biol Chem. 2014 Sep 5;289(36):25276-86. doi: 10.1074/jbc.M114.575985. Epub 2014 Jul 7.


The abundance and functional activity of proteins involved in the formation of the SNARE complex are tightly regulated for efficient exocytosis. Tomosyn proteins are negative regulators of exocytosis. Tomosyn causes an attenuation of insulin secretion by limiting the formation of the SNARE complex. We hypothesized that glucose-dependent stimulation of insulin secretion from β-cells must involve reversing the inhibitory action of tomosyn. Here, we show that glucose increases tomosyn protein turnover. Within 1 h of exposure to 15 mM glucose, ~50% of tomosyn was degraded. The degradation of tomosyn in response to high glucose was blocked by inhibitors of the proteasomal pathway. Using (32)P labeling and mass spectrometry, we showed that tomosyn-2 is phosphorylated in response to high glucose, phorbol esters, and analogs of cAMP, all key insulin secretagogues. We identified 11 phosphorylation sites in tomosyn-2. Site-directed mutagenesis was used to generate phosphomimetic (Ser → Asp) and loss-of-function (Ser → Ala) mutants. The Ser → Asp mutant had enhanced protein turnover compared with the Ser → Ala mutant and wild type tomosyn-2. Additionally, the Ser → Asp tomosyn-2 mutant was ineffective at inhibiting insulin secretion. Using a proteomic screen for tomosyn-2-binding proteins, we identified Hrd-1, an E3-ubiquitin ligase. We showed that tomosyn-2 ubiquitination is increased by Hrd-1, and knockdown of Hrd-1 by short hairpin RNA resulted in increased abundance in tomosyn-2 protein levels. Taken together, our results reveal a mechanism by which enhanced phosphorylation of a negative regulator of secretion, tomosyn-2, in response to insulin secretagogues targets it to degradation by the Hrd-1 E3-ubiquitin ligase.

Keywords: Diabetes; Insulin; Insulin Secretion; SNARE Proteins; Type 2 Diabetes.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Adaptor Proteins, Vesicular Transport
  • Animals
  • Binding Sites / genetics
  • Cell Line, Tumor
  • Cells, Cultured
  • Glucose / pharmacology
  • HEK293 Cells
  • Humans
  • Immunoblotting
  • Insulin / metabolism*
  • Insulin Secretion
  • Insulin-Secreting Cells / drug effects
  • Insulin-Secreting Cells / metabolism*
  • Mice
  • Models, Molecular
  • Mutation
  • Phosphorylation / drug effects
  • Protein Binding
  • Protein Structure, Tertiary
  • Proteolysis / drug effects
  • R-SNARE Proteins / chemistry
  • R-SNARE Proteins / genetics
  • R-SNARE Proteins / metabolism*
  • RNA Interference
  • Serine / chemistry
  • Serine / genetics
  • Serine / metabolism*
  • Ubiquitin-Protein Ligases / genetics
  • Ubiquitin-Protein Ligases / metabolism
  • Ubiquitination / drug effects


  • Adaptor Proteins, Vesicular Transport
  • Insulin
  • R-SNARE Proteins
  • STXBP5L protein, mouse
  • Serine
  • SYVN1 protein, human
  • Ubiquitin-Protein Ligases
  • Glucose