The mammalian guanine nucleotide exchange factor mSec12 is essential for activation of the Sar1 GTPase directing endoplasmic reticulum export

Traffic. 2001 Jul;2(7):465-75. doi: 10.1034/j.1600-0854.2001.20704.x.


The Sar1 GTPase is an essential component of COPII vesicle coats involved in export of cargo from the endoplasmic reticulum of mammalian cells. To begin to elucidate its mechanism of action, we now report the identity of the mammalian homolog to the yeast Sec12 guanine nucleotide exchange factor (18% identity) that promotes Sar1 activation. Mammalian Sec12 (mSec12) is a type II transmembrane protein with a large cytosolic domain, a fragment of which has previously been reported as the transcription factor prolactin regulatory element binding protein (PREB). mSec12 promotes efficient guanine nucleotide exchange on Sar1, but not Arf1 or Rab GTPases. mSec12 is localized to the endoplasmic reticulum and an antibody to the cytosolic domain of mSec12 potently inhibits Sar1 recruitment and the formation of COPII vesicles in vitro. The dominant negative GDP-restricted mutant Sar1[T39N] is shown to be a potent inhibitor of mSec12 activity, consistent with its role in preventing COPII vesicle formation in vitro and during transient expression in vivo. We propose that mSec12 is an evolutionarily distant guanine nucleotide exchange factor directing Sar1 GTPase activation in mammalian cells. Its divergence from yeast Sec12p may reflect the specialized needs of the mammalian endoplasmic reticulum involving the formation of Sar1-dependent transitional elements (Aridor M, et al. J Cell Biol 2001;152:213-229) and selection of cargo into prebudding complexes.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • ADP-Ribosylation Factor 1 / metabolism
  • Amino Acid Sequence
  • Animals
  • Arabidopsis / metabolism
  • Biological Evolution
  • Caenorhabditis elegans / metabolism
  • Cloning, Molecular
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism*
  • Endoplasmic Reticulum / metabolism
  • Guanine Nucleotide Exchange Factors / chemistry
  • Guanine Nucleotide Exchange Factors / genetics
  • Guanine Nucleotide Exchange Factors / metabolism*
  • Immunoglobulin G
  • Mammals
  • Mice
  • Molecular Sequence Data
  • Monomeric GTP-Binding Proteins / chemistry
  • Monomeric GTP-Binding Proteins / genetics
  • Monomeric GTP-Binding Proteins / metabolism*
  • Protein Transport
  • Rabbits
  • Recombinant Fusion Proteins / isolation & purification
  • Recombinant Fusion Proteins / metabolism
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / metabolism
  • Saccharomyces cerevisiae Proteins*
  • Sequence Alignment
  • Sequence Homology, Amino Acid
  • Transcription Factors / genetics
  • Transcription Factors / metabolism*
  • Vesicular Transport Proteins


  • DNA-Binding Proteins
  • Guanine Nucleotide Exchange Factors
  • Immunoglobulin G
  • Preb protein, mouse
  • Recombinant Fusion Proteins
  • Saccharomyces cerevisiae Proteins
  • Transcription Factors
  • Vesicular Transport Proteins
  • ADP-Ribosylation Factor 1
  • Monomeric GTP-Binding Proteins
  • SAR1 protein, S cerevisiae