ANALYSIS OF THE LOCALIZATION OF STE6 CFTR CHIMERAS IN A SACCHAROMYCES-CEREVISIAE MODEL FOR THE CYSTIC-FIBROSIS DEFECT CFTR-DELTA-F508/

The use of yeast as a model system to study mammalian proteins is attr active, because yeast genetic tools can be utilized if a suitable phen otype is created. STE6, the Saccharomyces cerevisiae a-factor mating p heromone transporter, and CFTR, the mammalian cystic fibrosis transmem brane conductance regulator, are both members of the ATP binding casse tte (ABC) superfamily. Teem et al, (1993) described a yeast model for studying a mutant form of the cystic fibrosis protein, CFTR Delta F508 . The model involved expression of a chimeric molecule in which a port ion of yeast STE6 was replaced with the corresponding region from mamm alian CFTR. The STE6/CFTR chimera complemented a ste6 mutant strain fo r mating, indicating that it could export a-factor. However, mating ef ficiency was dramatically reduced upon introduction of Delta F508, pro viding a yeast phenotype for this mutation, In human cells, the Delta F508 mutation results in retention of CFTR in the endoplasmic reticulu m (ER), and possibly in reduction of its chloride-channel activity, He re we examine the basis for the differences in STE6 activity promoted by the wild-type and mutant STE6/CFTR chimeras, By analysis of protein stability and subcellular localization, we find that the mutant chime ra is not ER-retained in yeast, We conclude that the molecular basis f or the reduced mating of the STE6/CFTR Delta F508 chimera must reflect a reduction in its capacity to transport a-factor, rather than mistra fficking. Thus, STE6/CFTR Delta F508 in yeast appears to be a good gen etic model to probe certain aspects of protein function, but not to st udy protein localization.