REGULATION OF INOSITOL TRANSPORT IN SACCHAROMYCES-CEREVISIAE INVOLVESINOSITOL-INDUCED CHANGES IN PERMEASE STABILITY AND ENDOCYTIC DEGRADATION IN THE VACUOLE

Uptake of inositol by Saccharomyces cerevisiae is mediated by a specif ic inositol permease encoded by the ITR1 gene, Removal of inositol fro m the growth medium results in an increase in ITR1 mRNA abundance, The increase in ITR1 mRNA is accompanied by an increase in de novo synthe sis of the Itr1 permease leading to an increased capacity for uptake, When inositol is added to the growth medium inactivation of uptake act ivity occurs, and both transcription of ITR1 and uptake activity are r epressed to a basal level of function, The transcriptional regulation of ITR1 depends on the INO2, INO4, and OPI1 genes, In addition, repres sion is also achieved by regulation of ITR1 expression at the post-tra nslational level, In this study, we show that there is a change in the stability of the Itr1 permease after the addition of inositol to the growth medium, Immunoblot analysis using a monoclonal antibody against an epitope attached to the Itr1 permease showed that the addition of inositol causes a dramatic increase in the rate of degradation of the permease, After the repressed (basal) level is achieved, turnover cont inues to be rapid, The increased rate of degradation was also observed in strains with mutations that block conjugation to ubiquitin, Degrad ation was not observed in strains defective in the END3/END4 endocytic pathway or in the production of vacuolar proteases (PEP4). Thus, inac tivation of the Itr1 permease is accompanied by endocytic internalizat ion followed by degradation in the vacuole, Inactivation may be a sepa rate process that precedes and signals endocytic degradation, Since th e end3/end4 mutations did not affect uptake activity under derepressed conditions, endocytosis is not required for normal inositol uptake.