Mutational Disruption of Plasma Membrane Trafficking of Saccharomyces cerevisiae Yor1p, a Homologue of Mammalian Multidrug Resistance Protein

Abstract

The ATP binding cassette (ABC) transporter protein Yor1p was identified on the basis of its ability to elevate oligomycin resistance when it was overproduced from a high-copy-number plasmid. Analysis of the predicted amino acid sequence of Yor1p indicated that this protein was a new member of a subfamily of ABC transporter proteins defined by the multidrug resistance protein (MRP). In this work, Yor1p is demonstrated to localize to the Saccharomyces cerevisiaeplasma membrane by both indirect immunofluorescence and biochemical fractionation studies. Several mutations were generated in the amino-terminal nucleotide binding domain (NBD1) of Yor1p to test if the high degree of sequence conservation in this region of the protein was important for function. Deletion of a phenylalanine residue at Yor1p position 670 led to a mutant protein that appeared to be retained in the endoplasmic reticulum (ER) and that was unstable. As shown by others, deletion of the analogous residue from a second mammalian MRP family member, the cystic fibrosis transmembrane conductance regulator (CFTR), also led to retention of this normally plasma membrane-localized protein in the ER. Changes in the spacing between or the sequences flanking functional motifs of Yor1p NBD1 led to defective trafficking or decreased activity of the mutant proteins. Analyses of the degradation of wild-type and ΔF670 Yor1p indicated that the half-life of ΔF670 Yor1p was dramatically shortened. While the vacuole was the primary site for turnover of wild-type Yor1p, degradation of ΔF670 Yor1p was found to be more complex with both proteasomal and vacuolar contributions.

ACKNOWLEDGMENTS

We thank Rob Piper, Mark Stamnes, Scott Emr, Mark Hochstrasser, Dieter Wolf, Karl Kuchler, Chris Kaiser, and Ralf Kölling for discussions and materials. Antibodies were provided by AndréGoffeau, Tom Rapaport, Colin Stirling, Rob Piper, and Mark Rose. The ubc7 disruption plasmid was provided by Manfred Koegl and Stephan Jentsch. Thanks go to Rob Piper for a critical reading of the manuscript.

This work was supported in part by NIH grants GM49825 (W.S.M.-R.) and DK25295 (University of Iowa Diabetes and Endocrinology Research Center). W.S.M.-R. is an established investigator of the American Heart Association.