Disease relevance of SEC18

• Yeast Sec18p and mammalian N-ethylmaleimide-sensitive fusion protein, which are involved in intracellular transport, yeast Pas1p, which is essential for peroxisome assembly, and mammalian TBP-1, which influences HIV gene expression, are 40% identical in the duplicated region [1].

High impact information on SEC18

• Chaperones (NSF/Sec18p, LMA1, and -SNAP/Sec17p) disassemble cis-SNARE complexes to prepare for the docking of organelles rather than to drive fusion [2].
• LMA1 binds to vacuoles at Sec18p (NSF), transfers upon ATP hydrolysis to a t-SNARE (Vam3p) complex, and is released during fusion [3].
• Upon Sec18p ATP hydrolysis, LMA1 transfers to (and stabilizes) a Vam3p complex [3].
• Here we show that Cdc48p, a cell-cycle protein with homology to Sec18p, is required in ER fusion [4].
• By electron microscopy three of the mutants, sec18, sec17, and sec22, accumulate 50 nm vesicles at the nonpermissive temperature [5].

Biological context of SEC18

• SEC18/NSF-independent, protein-sorting pathway from the yeast cortical ER to the plasma membrane [6].
• Although potential sites for N-linked glycosylation were present in the Sec18p sequence, the sizes of the in vivo SEC18 gene products were unaffected by the drug tunicamycin, indicating that Sec18p does not enter the secretory pathway [7].
• Gene disruption has shown that SEC18 is essential for yeast cell growth [7].
• In the case of SEC18, DNA sequence analysis has demonstrated homology to a factor needed for transport in mammalian in vitro systems [8].
• This involved random mutagenesis of a GAL1-regulated SEC18 yeast expression plasmid [9].

Anatomical context of SEC18

• By cloning and sequencing its gene from Chinese hamster ovary cells and by use of in vitro assays, we show that this fusion protein is equivalent to the SEC18 gene product of the yeast Saccharomyces cerevisiae, known to be essential for vesicle-mediated transport from the endoplasmic reticulum to the Golgi apparatus [10].
• SEC18 gene function is required for secretory protein transport between the endoplasmic reticulum (ER) and the Golgi complex [7].
• A reversible delay in Sec18p action prevents vacuoles from acquiring resistance to any of these three drugs, confirming that their action is on the normal fusion pathway [11].
• Electron microscopy shows that Golgi membranes vesiculate in sec18 cells within 10 min of a temperature shift [12].
• Purified Sec18p, Uso1p, and LMA1 in the presence of ATP and GTP satisfies the requirement for cytosol in fusion of ER-derived vesicles with Golgi membranes [13].

Associations of SEC18 with chemical compounds

• Ergosterol is required for the Sec18/ATP-dependent priming step of homotypic vacuole fusion [11].
• First, we showed that Sec18p (N-ethylmaleimide-sensitive fusion protein, NSF) and Vti1p (soluble N-ethylmaleimide-sensitive fusion protein attachment protein, SNARE), and soluble N-ethylmaleimide-sensitive fusion protein receptor are required for fusion of the autophagosome to the vacuole but are not involved in autophagosome formation [14].
• The vesicles produced are functionally distinct from the ER: they transfer pro-alpha-factor to the Golgi apparatus faster and more efficiently than the ER, they do not require Sec12p or Sec23p to complete transfer, and transfer is resistant to GTP gamma S. Targeting of vesicles to the Golgi apparatus requires Ypt1p and Sec18p [15].
• When the exit of vesicles from the endoplasmic reticulum or entry into the cis-Golgi were blocked in sec12 or sec18 cells, all anchors contained a side chain consisting of a single alpha 1,2-linked mannose [16].
• We show that Vac8 acylation is restricted to a narrow time window, is independent of ATP hydrolysis by Sec18, and is stimulated by the ion chelator EDTA [17].

Physical interactions of SEC18

• Vam7p is a constituent of the vacuole SNARE complex and is released from this complex by the Sec17p/Sec18p/ATP-mediated priming of the vacuoles [18].
• Ordering experiments using the dilution resistant intermediate and reversible Sec23p complex inhibition indicate Sec18p action is required before LMA1 function [13].

Regulatory relationships of SEC18

• Strikingly, palmitoylation of Vac8p is blocked by the addition of antibodies to Sec18p (yeast NSF) only [19].

Other interactions of SEC18

• The vacuole v-t-SNARE complex is disassembled by Sec17p/alpha-SNAP and Sec18p/NSF prior to vacuole docking and fusion [18].
• Vam10p defines a Sec18p-independent step of priming that allows yeast vacuole tethering [20].
• The two tRNA genes, coding for a tRNA(asp) and a tRNA(arg), and three of the ORFs, had been sequenced previously, i.e. HSP26, SEC18, and UBC4 [21].
• Upon priming by Sec18p/NSF and ATP, Vam2/6p is released as a 38S subcomplex that binds Ypt7p to initiate docking [22].
• In this study, we show that the COOH-terminal domain of Ist2p is necessary and sufficient to mediate SEC18-independent sorting when it is positioned at the COOH terminus of different integral membrane proteins and exposed to the cytoplasm [6].

Analytical, diagnostic and therapeutic context of SEC18

• We have determined the structure of the N-terminal domain from Sec18p by x-ray crystallography [23].
• While preliminary cell fractionation studies showed that Sec18p is not associated with the ER or Golgi complex, association with a 100,000 x g pellet fraction was observed [7].
• Immunofluorescence microscopy with the anti-Sar1p antibody showed perinuclear staining that was exaggerated in the ER-accumulating sec18 mutant [24].

References