Disease relevance of SEC4

• In the present study, purified recombinant Sec4 and Ypt1 proteins expressed in E. coli have been used as substrates to determine if GTPase activating proteins (GAPs) directed toward these proteins are present in rat pancreas [1].

High impact information on SEC4

• A functional fusion of Sec3 protein with green fluorescent protein (Sec3-GFP) localizes to the site of polarized exocytosis for each cell-cycle stage, where it colocalizes with Sec4p and Sec8p [2].
• Sec9 is a SNAP-25-like component of a yeast SNARE complex that may be the effector of Sec4 function in exocytosis [3].
• A set of 11 clones encoding putative GTP binding proteins highly homologous to the yeast YPT1/SEC4 gene products have been isolated from an MDCK cell cDNA library [4].
• In all post-Golgi blocked sec mutants, Sec4p is predominantly associated with the secretory vesicles that accumulate as a result of the secretory block [5].
• Sec4p is synthesized as a soluble protein that rapidly (t1/2 less than or equal to 1 min) and tightly associates with secretory vesicles and the plasma membrane by virtue of a conformational change of a covalent modification [5].

Biological context of SEC4

• All phenotypes of the temperature-sensitive sec2 alleles are partially suppressed by duplication of the SEC4 gene, but the lethality of a sec2 disruption is not suppressed [6].
• SEC4 is an essential gene encoding a small GTPase that is involved in Golgi to cell surface transport in Saccharomyces cerevisiae and is a paradigm for studies on the mode of action of Rab proteins [7].
• Sec8p and Sec15p are components of a plasma membrane-associated 19.5S particle that may function downstream of Sec4p to control exocytosis [8].
• On the amino acid sequence level, and particularly with respect to known small G protein specificity domains, YptV1p and YptC1p are the closest algal analogs of yeast Ypt1p, with 70% overall identity and identical effector regions, but YptV2p is only 55% identical to Ypt1p, and its effector domain resembles that of Sec4p [9].
• To study the regulation of membrane traffic in epithelial cells, a set of 11 clones encoding proteins similar to the YPT1/SEC4 products were isolated from an MDCK (Madin-Darby canine kidney) cell cDNA library [10].

Anatomical context of SEC4

• cDNAs representing nine small G protein genes encoding Ypt proteins from the green algae Volvox carteri (YptV) and Chlamydomonas reinhardtii (YptC) were tested for their ability to complement mutations in the YPT1, SEC4, and YPT7 genes of Saccharomyces cerevisiae strains defective in different steps of intracellular vesicle transport [9].
• The proper localization of Sec2p to secretory vesicles is essential for its function and is largely independent of Sec4p [11].
• It encodes a protein of 759 amino acids that functions as a guanine nucleotide exchange factor for the small GTPase Sec4p, a regulator of Golgi to plasma membrane transport [11].
• Dss4p is found both on membranes and in the cytosol; however, it is the membrane fraction that is complexed to Sec4p [7].
• Small GTP-binding proteins of the YPT/SEC4/Rab family have been shown to play an essential role in intracellular membrane trafficking [12].

Associations of SEC4 with chemical compounds

• The Sec2p NH(2) terminus encodes an exchange factor for the Rab protein Sec4p [13].
• None of the D. salina GTP-binding proteins reacted positively with polyclonal antibodies raised against SEC4, rab1 or rab6 proteins [14].
• In mammalian cells the effects of a non-hydrolysable GTP analogue, GTP-gamma S, on different transport events have suggested that they also have proteins functionally related to yeast Sec4p and Ypt1p [15].
• Here we show that the yeast Ypt1 and Sec4 proteins, essential components of the vesicular transport machinery, allow unimpaired vesicular transport when permanently fixed to membranes by membrane-spanning domains replacing their two C-terminal cysteine residues [16].
• To test the role of GTP hydrolysis in Sec4 function, we constructed a mutation, Q-79----L, analogous to the oncogenic mutation of Q-61----L in Ras, in a region of Sec4 predicted to interact with the phosphoryl group of GTP [17].

Regulatory relationships of SEC4

• The S.pombe ypt2 gene under control of the S.cerevisiae GAL10 promoter is able to suppress the temperature-sensitive phenotype of a S. cerevisiae sec4 mutant, indicating a functional similarity of these GTP-binding proteins from the two very distantly related yeasts [18].
• Overexpression of Mso1 lacking this domain (Mso1-(1-193)) inhibited the growth of cells bearing an attenuated Sec4 GTPase [19].
• GAL1-regulated expression plasmids carrying the mutant sec4 allele (pS28N) had minimal effects in glucose-incubated C. albicans transformants, but six of nine transformants tested grew very slowly in galactose [20].

Other interactions of SEC4

• The genes SEC4 and YPT1 encode Ras-related GTP-binding proteins in the yeast Saccharomyces cerevisiae [21].
• Mutation of SEC1, SEC4, or SEC8 blocked spore formation, and electron microscopic analysis of the sec4-8 mutant indicated that this inability to produce spores was caused by a failure to form the prospore membrane [22].
• However, electron microscopy revealed that while sec4 mutant cells accumulate secretory vesicles, ypt31/32 mutant cells accumulate aberrant Golgi structures [23].
• Temperature-sensitive (ts) mutations in sec2 and sec4 result in a tight block in secretion and the accumulation of secretory vesicles randomly distributed in the cell [11].
• Ypt32 recruits the Sec4p guanine nucleotide exchange factor, Sec2p, to secretory vesicles; evidence for a Rab cascade in yeast [11].

Analytical, diagnostic and therapeutic context of SEC4

• Gel filtration studies suggest that a portion of Sec4p is in association with the Sec8p/Sec15p particle [8].
• Molecular cloning of YPT1/SEC4-related cDNAs from an epithelial cell line [10].
• One chromosomal SEC4 allele in C. albicans CAI4 was readily disrupted by homologous gene targeting, but efforts to disrupt the second allele yielded no viable null mutants [20].

References