High impact information on SEC53

• Myoinositol gets incorporated into numerous membrane glycoproteins of Saccharomyces cerevisiae; incorporation is dependent on phosphomannomutase (sec53) [1].
• These results suggested that the Sec53 protein does not function directly in the translocation and glycosylation of prepro-alpha-factor [2].
• To examine this point further, we isolated membranes from sec53 cells that had been grown at the non-permissive temperature prior to disruption [2].
• Analysis of sec53 presented an opportunity not only to see if mutants could be examined in recently developed yeast in vitro translocation systems, but also to characterize further the nature of this mutant originally postulated to be defective in protein translocation [2].
• The in vitro phenotype of sec53 could be mimicked by isolating rough microsomes from wild-type cells that had been grown for 1 h in the presence of tunicamycin [2].

Biological context of SEC53

• This prediction is confirmed by detection of a 28-kD protein overproduced in cells that carry SEC53 on a multicopy plasmid [3].
• In the yeast Saccharomyces cerevisiae PMM is encoded by the gene SEC53 (ScSEC53) and the deficiency of PMM activity leads to severe defects in both protein glycosylation and secretion [4].
• Assembly of Lipo-EtxB was found to be unaffected in a sec18 mutant of S. cerevisiae, which possesses a temperature-sensitive defect in protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus, but was found not to assemble in a sec53 mutant, which causes the misfolding of proteins targeted to the ER [5].
• To follow Sec53p more directly, a LacZ-SEC53 gene fusion has been constructed which allows the isolation of a hybrid protein for use in production of antibody [3].
• The residual phosphomannomutase activity in mutant cell lysates is thermolabile in proportion to the severity of the sec53 cell growth defect [6].

Anatomical context of SEC53

• SEC53, a gene that is required for completion of assembly of proteins in the endoplasmic reticulum in yeast, has been cloned, sequenced, and the product localized by cell fractionation [3].
• Cell fractionation, performed with conditions that preserve the lumenal content of the endoplasmic reticulum (ER), shows Sec53p highly enriched in the cytosol fraction [3].
• RESULTS: The patients have 30% to 50% normal PMM activity in fibroblasts due to different mutations in PMM2, and we studied the effect of each mutation on the PMM activity in a Saccharomyces cerevisiae expression system [7].

Associations of SEC53 with chemical compounds

• Membrane-bound Sec53 protein is largely insoluble in Triton X-100, but the protein is effectively released from the membrane by urea [8].
• The secretory defect in sec53 cells may now be explained by a deficit in GDP-mannose production [6].
• This pattern of stimulation suggests that extracts of sec53 cells are deficient in phosphomannomutase activity or in the production of a precursor of mannose 1-phosphate [6].
• Phosphomannomutase (PMM) is a key enzyme, which catalyses one of the first steps in the glycosylation pathway, the conversion of D-mannose-6-phosphate to D-mannose-1-phosphate [4].
• Reducing the PMM expression level through virus-induced gene silencing caused a substantial decrease in ascorbic acid (AsA) content in N. benthamiana leaves [9].

Other interactions of SEC53

• There was virtually no outer chain addition in the alg2 and alg4 mutants. alg4 was the only mutant that failed to secrete invertase [10].
• A functional, epitope-tagged Kre1p is shown to be highly modified in a SEC53-dependent manner [11].
• The nucleotide sequence of GGP1 predicts a 60-kDa polypeptide, in agreement with the molecular mass of the gp115 precursor detected in sec53 mutant cells at restrictive temperature [12].
• The fission yeast sns mutants arrest with a similar cell cycle block and interact genetically with the Ran system. sns-A10, sns-B2 and sns-B9 have mutations in the fission yeast homologues of S. cerevisiae Sar1p, Sec31p and Sec53p, respectively, which are required for the early steps of the protein secretory pathway [13].

Analytical, diagnostic and therapeutic context of SEC53

• Finally, the Sec53 protein and phosphomannomutase activity cofractionate exactly in a 70-fold partial purification involving gel filtration and DEAE chromatography [6].

References