Enhanced protein export in Saccharomyces cerevisiae nud1 mutants is an active process.

Saccharomyces cerevisiae NUD1 gene codes for a spindle pole body component and nud1 temperature-sensitive mutants arrest at 38 degrees C in late anaphase with a tendency for lysis. We found that addition of 10% sorbitol to the medium complemented the lytic phenotype, and determination of colony-forming units evidenced the viability of nud1 cells for at least 48 hours at 38 degrees C. The protein amount in cell-free medium increased at 38 degrees C, and evidence is presented that intact nud1 cells exported proteins in amounts 10-fold higher compared wild type strains. The observed high amounts of extracellular acid phosphatase, invertase, and bacterial beta-galactosidase suggested the export of secretory proteins. This was evidenced by construction of nudlsec mutants and the observation that interruption of the secretory pathway resulted in absence of protein export at 38 degrees C. Proteins were exported through a cell wall showing increased porosity at 38 degrees C. The extracellular release of Gas1p and the facilitated transformability with plasmid DNA of nud1 cells indicated alternations of their cell walls at 38 degrees C. The export of proteins depends on oxidative phosphorylation as evidenced by disruption of the COX10 gene. Experiments with inhibitors of mitochondrial functions showed that the synthesis of adenosine triphosphate, but not the electron transport along the respiratory chain, has a key role in the export of proteins. The data show that the phenotype of S. cerevisiae nud1 mutants is characterized by enhanced export of secretory proteins and that the passage of proteins through the walls of nud1 cells is an active process.