Mutations that alter both localization and production of a yeast nuclear protein

The first 74 amino acids of the yeast GAL4 gene product are sufficient to localize a GAL4-beta-galactosidase chimeric protein to the yeast nucleus. Chimeric proteins missing the first 74 GAL4 amino acids, but containing almost all of the rest of GAL4, are not localized to the nucleus and are expressed at higher levels than their nuclear counterparts. On this basis, point mutations within GAL4, which reduce nuclear localization and increase production of a normally nuclear GAL4-beta-galactosidase fusion protein, were isolated and sequenced. The effect of these mutations on the localization and expression of the intact GAL4 protein was examined. The degree to which the mutant proteins are excluded from the nucleus varies, but all mutations cause overproduction of the protein. Point mutations altering two of the six cysteine residues of the GAL4 putative 'zinc finger' abolish gene activation by intact GAL4; however, mutations in nearby residues have no effect on GAL4-dependent gene activation.