SEQUENCE DETERMINANTS FOR REGULATED DEGRADATION OF YEAST 3-HYDROXY-3-METHYLGLUTARYL-COA REDUCTASE, AN INTEGRAL ENDOPLASMIC-RETICULUM MEMBRANE-PROTEIN

The degradation rate of 3-hydroxy-3-methylglutaryl CoA reductase (HMG- R), a key enzyme of the mevalonate pathway, is regulated through a fee dback mechanism by the mevalonate pathway. To discover the intrinsic d eterminants involved in the regulated degradation of the yeast HMG-R i sozyme Hmg2p, we replaced small regions of the Hmg2p transmembrane dom ain with the corresponding regions from the other, stable yeast HMG-R isozyme Hmg1p. When the first 26 amino acids of Hmg2p were replaced wi th the same region from Hmg1p, Hmg2p was stabilized. The stability of this mutant was not due to mislocalization, but rather to an inability to be recognized for degradation. When amino acid residues 27-54 of H mg2p were replaced with those from Hmg1p, the mutant was still degrade d, but its degradation rate was poorly regulated. The degradation of t his mutant was still dependent on the first 26 amino acid residues and on the function of the HRD genes. These mutants showed altered ubiqui tination levels that were well correlated with their degradative pheno types. Neither determinant was sufficient to impart regulated degradat ion to Hmg1p. These studies provide evidence that there are sequence d eterminants in Hmg2p necessary for degradation and optimal regulation, and that independent processes may be involved in Hmg2p degradation a nd its regulation.