Small GTPase Sar1 is crucial for proglutelin and α-globulin export from the endoplasmic reticulum in rice endosperm

J Exp Bot. 2013 Jul;64(10):2831-45. doi: 10.1093/jxb/ert128. Epub 2013 May 16.


Rice seed storage proteins glutelin and α-globulin are synthesized in the endoplasmic reticulum (ER) and deposited in protein storage vacuoles (PSVs). Sar1, a small GTPase, acts as a molecular switch to regulate the assembly of coat protein complex II, which exports secretory protein from the ER to the Golgi apparatus. To reveal the route by which glutelin and α-globulin exit the ER, four putative Sar1 genes (OsSar1a/b/c/d) were cloned from rice, and transgenic rice were generated with Sar1 overexpressed or suppressed by RNA interference (RNAi) specifically in the endosperm under the control of the rice glutelin promoter. Overexpression or suppression of any OsSar1 did not alter the phenotype. However, simultaneous knockdown of OsSar1a/b/c resulted in floury and shrunken seeds, with an increased level of glutelin precursor and decreased level of the mature α- and β-subunit. OsSar1abc RNAi endosperm generated numerous, spherical, novel protein bodies with highly electron-dense matrixes containing both glutelin and α-globulin. Notably, the novel protein bodies were surrounded by ribosomes, showing that they were derived from the ER. Some of the ER-derived dense protein bodies were attached to a blebbing structure containing prolamin. These results indicated that OsSar1a/b/c play a crucial role in storage proteins exiting from the ER, with functional redundancy in rice endosperm, and glutelin and α-globulin transported together from the ER to the Golgi apparatus by a pathway mediated by coat protein complex II.

Keywords: Endoplasmic reticulum; OsSar1; protein body; protein intracellular transport; protein storage vacuole; rice storage proteins..

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alpha-Globulins / genetics
  • Alpha-Globulins / metabolism*
  • Amino Acid Sequence
  • Endoplasmic Reticulum / genetics
  • Endoplasmic Reticulum / metabolism*
  • Endosperm / genetics
  • Endosperm / metabolism*
  • Glutens / genetics
  • Glutens / metabolism*
  • Molecular Sequence Data
  • Monomeric GTP-Binding Proteins / chemistry
  • Monomeric GTP-Binding Proteins / genetics
  • Monomeric GTP-Binding Proteins / metabolism*
  • Oryza / classification
  • Oryza / enzymology*
  • Oryza / genetics
  • Oryza / metabolism
  • Phylogeny
  • Plant Proteins / chemistry
  • Plant Proteins / genetics
  • Plant Proteins / metabolism*
  • Protein Precursors / genetics
  • Protein Precursors / metabolism
  • Protein Transport
  • Seeds / genetics
  • Seeds / metabolism
  • Sequence Alignment


  • Alpha-Globulins
  • Plant Proteins
  • Protein Precursors
  • Glutens
  • Monomeric GTP-Binding Proteins