title: Sequence of the M28 dsRNA: preprotoxin is processed to an alpha/beta heterodimeric protein toxin.
abstract: The killer and immunity phenotypes of K28 killer strains of Saccharomyces cerevisiae are determined by the 1.75-kb M28 dsRNA virus. In the plus strand, M28p, the K28 preprotoxin gene, comprises bases 13-1047 and is followed, after an additional 85 bases, by a 63-bp poly(A) sequence and a 553-base 3'-sequence. This 3'-sequence contains two potential stem-loop structures predicted to bind the L-A encoded cap-pol protein, initiating encapsidation; high-level expression results in curing of M1 dsRNA. Expression of M28p confers the complete K28 killer and immunity phenotype on a cell lacking M28 dsRNA. K28 toxin is a disulfide-bonded heterodimer of alpha (10.5 kDa) and beta (11 kDa) components whose N-termini correspond to M28p residues 50-61 and 246-257, respectively. alpha is preceded by a potentially redundant pair of secretion signal peptides; deletion of the first reduces toxin secretion by 75%. While M28p bears no sequence similarity to M1p, the K1 preprotoxin, the predicted patterns of processing by glycosylation and cleavage are remarkably similar. The beta N- and C-termini are probably processed by Kex2p and Kex1p, respectively; the mechanism of cleavage at the less typical sites bounding the alpha component is under investigation. While a kex2 delta mutation prevents toxin secretion, secreted toxin retains 20% activity in a kex1 delta mutant. Neither mutation affects immunity.
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002268:Carboxypeptidases / Q000378:metabolism; D017931:DNA Primers; D018076:DNA, Complementary; D010450:Endopeptidases / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D015966:Gene Expression Regulation, Fungal; D006031:Glycosylation; D055646:Killer Factors, Yeast; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D009183:Mycotoxins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D043484:Proprotein Convertases; D011499:Protein Processing, Post-Translational; D012330:RNA, Double-Stranded / Q000235:genetics*; D012331:RNA, Fungal / Q000235:genetics*; D012441:Saccharomyces cerevisiae / Q000235:genetics*; D029701:Saccharomyces cerevisiae Proteins; D012697:Serine Endopeptidases; D013381:Subtilisins / Q000378:metabolism
chemical_list: D017931:DNA Primers; D018076:DNA, Complementary; D005656:Fungal Proteins; D055646:Killer Factors, Yeast; D008565:Membrane Proteins; D009183:Mycotoxins; D012330:RNA, Double-Stranded; D012331:RNA, Fungal; D029701:Saccharomyces cerevisiae Proteins; D002268:Carboxypeptidases; D010450:Endopeptidases; C078874:carboxypeptidase D; D043484:Proprotein Convertases; D012697:Serine Endopeptidases; D013381:Subtilisins; C058751:KEX2 protein, S cerevisiae; C026515:type I signal peptidase