An MF alpha 1-SUC2 (alpha-factor-invertase) gene fusion for study of protein localization and gene expression in yeast

An immunological method for detecting gene expression in yeast colonies

A method for detection of cloned, expressed genes in yeast colonies has been developed. The 70-kilodalton (kDa) mitochondrial outer membrane protein of yeast was used as a model protein. Transformation of a strain deficient in the gene for the 70-kDa protein was performed, and transformed colonies were detected with the antibody decoration technique. This technique is based upon gentle lysis of yeast colonies that have been grown on nitrocellulose filters such that the yeast proteins remain bound in discrete spots after lysis. The lysis is carried out by alkaline conditions in the presence of 2-mercaptoethanol and sodium dodecyl sulfate. After lysis, empty sites on the nitrocellulose filter are blocked to eliminate nonspecific binding of proteins by either 0.5% bovine serum albumin or 0.05% Tween 20. Decoration with antibody is visualized by using 125I-labeled protein A or peroxidase-conjugated second antibody. Antigens amounting to less than 0.1% of the total protein in the cell can be readily detected by the assay. The sensitivity of the assay enables detection of 1 positive colony per plate containing about 1000 colonies.

Invertase beta-galactosidase hybrid proteins fail to be transported from the endoplasmic reticulum in Saccharomyces cerevisiae

The yeast SUC2 gene codes for the secreted enzyme invertase. A series of 16 different-sized gene fusions have been constructed between this yeast gene and the Escherichia coli lacZ gene, which codes for the cytoplasmic enzyme beta-galactosidase. Various amounts of SUC2 NH2-terminal coding sequence have been fused in frame to a constant COOH-terminal coding segment of the lacZ gene, resulting in the synthesis of hybrid invertase-beta-galactosidase proteins in Saccharomyces cerevisiae. The hybrid proteins exhibit beta-galactosidase activity, and they are recognized specifically by antisera directed against either invertase or beta-galactosidase. Expression of beta-galactosidase activity is regulated in a manner similar to that observed for invertase activity expressed from a wild-type SUC2 gene: repressed in high-glucose medium and derepressed in low-glucose medium. Unlike wild-type invertase, however, the invertase-beta-galactosidase hybrid proteins are not secreted. Rather, they appear to remain trapped at a very early stage of secretory protein transit: insertion into the endoplasmic reticulum (ER). The hybrid proteins appear only to have undergone core glycosylation, an ER process, and do not receive the additional glycosyl modifications that take place in the Golgi complex. Even those hybrid proteins containing only a short segment of invertase sequences at the NH2 terminus are glycosylated, suggesting that no extensive folding of the invertase polypeptide is required before initiation of transmembrane transfer. beta-Galactosidase activity expressed by the SUC2-lacZ gene fusions cofractionates on Percoll density gradients with ER marker enzymes and not with other organelles. In addition, the hybrid proteins are not accessible to cell-surface labeling by 125I. Accumulation of the invertase-beta-galactosidase hybrid proteins within the ER does not appear to confer a growth-defective phenotype to yeast cells. In this location, however, the hybrid proteins and the beta-galactosidase activity they exhibit could provide a useful biochemical tag for yeast ER membranes.

Secretion of foreign proteins from Saccharomyces cerevisiae directed by alpha-factor gene fusions

Fusions between the cloned yeast alpha-factor structural gene and chemically synthesized DNA segments encoding human protein analogs have been constructed. The gene fusions encode hybrid proteins that include the first 89 amino acids of the native alpha-factor precursor fused to either a small (beta-endorphin, 31 amino acids) or large (alpha-interferon, 166 amino acids) foreign protein. Proteolytic cleavage sites involved in alpha-factor maturation from the native precursor immediately precede the foreign peptide in the hybrid protein. The alpha-factor promoter was utilized to express the gene fusions in Saccharomyces cerevisiae and resulted in the efficient secretion of the foreign proteins into the culture medium. The processing of the hybrid proteins has been characterized by amino acid sequence analysis of the secreted proteins. The proteolytic cleavages involved in the maturation of alpha-factor peptides from the native precursor also occur accurately in the hybrid protein. In addition, cleavages occurred on the carboxyl side of two lysines within the beta-endorphin peptide. Internal cleavages in the interferon protein were also detected. However, in this case, the cleavages occurred at a very low frequency such that greater than 95% of the secreted interferon remained intact.

Alpha-factor-directed synthesis and secretion of mature foreign proteins in Saccharomyces cerevisiae

Saccharomyces cerevisiae cells were transformed with plasmids containing hybrid genes in which the sequence encoding mature human epidermal growth factor was joined to sequences encoding the leader region (preprosegment) of the precursor of the yeast mating pheromone alpha-factor. These cells accurately process the hybrid protein and efficiently secrete authentic biologically active human epidermal growth factor into the medium.

Intracellular targeting and import of an F1-ATPase beta-subunit-beta-galactosidase hybrid protein into yeast mitochondria

The gene coding for the yeast mitochondrial F1-ATPase beta subunit (ATP2) has been fused to the Escherichia coli lacZ gene. The chimeric ATP2-lacZ gene codes for a hybrid protein consisting of some 350 amino acids of the F1-ATPase beta subunit at its amino terminus and a large enzymatically active portion of the lacZ gene product, beta-galactosidase (beta-D-galactoside galactohydrolase, EC 3.2.1.23), at its carboxyl terminus. The beta-subunit-beta-galactosidase hybrid protein is expressed in both E. coli and yeast. In yeast, this hybrid molecule is targeted to the mitochondrion and is protected in isolated mitochondria from added protease under conditions in which an outer membrane enzymatic marker is digested. Yeast cells carrying the ATP2-lacZ gene fusion on plasmid p beta Z1 are unable to grow on a nonfermentable carbon source. Upon loss of the p beta Z1 plasmid, growth of the cured host strain on the nonfermentable substrate is restored. In the presence of the beta-subunit-beta-galactosidase hybrid protein, the energy-transducing capacity of the mitochondrial membrane as measured by the 32Pi-ATP exchange reaction is only 9% of that measured in the absence of the gene fusion product. The results indicate that it is the presence of the beta-subunit-beta-galactosidase hybrid protein within mitochondria that interferes with function(s) essential for respiratory growth. These observations open up the prospect of genetic characterization of the signals and cellular machinery responsible for mitochondrial protein delivery.