Q2N and E64G double mutation of ubiquitin confers a stress sensitive phenotype on Saccharomyces cerevisiae.

ABSTRACT

The eukaryotic protein, ubiquitin harbours a parallel β-bulge in its structure which is formed by residues Glu64(1), Ser65(2) and Gln2(X). Despite their low % frequency of occurrence in parallel β-bulges, the residues Gln2 and Glu64 have been totally conserved in ubiquitin. In a previous study, two single mutants UbQ2N and UbE64G were constructed by replacing the residues Gln2 and Glu64 with Asn and Gly, respectively to understand their importance. The choice of the residues for substitution was made on the basis of their high preference for existence in parallel β-bulge, so that the structure of mutants remains unaltered and any functional differences observed would highlight the importance of Gln2 and Glu64 in ubiquitin biology. The results from this study established that yeast cells expressing either UbQ2N or UbE64G, displayed functional differences with respect to survival upon exposure to cycloheximide and degradation of substrates by ubiquitin fusion degradation (UFD) pathway. It describes construction of the double mutant UbQ2N-E64G and its characterization. Our results showed expression of UbQ2N-E64G in stress hypersensitive SUB60 cells led to significant decrease in growth rate and prolonged half-life of substrates of UFD pathway, besides failure of complementation under heat and antibiotic stresses, providing the reason for conservation of Gln2 and Glu64 in ubiquitin sequence.