Duane D Jenness PhD
|Institution||University of Massachusetts Medical School|
|Department||Microbiology and Physiological Systems|
|Address||University of Massachusetts Medical School|
55 Lake Avenue North
Worcester MA 01655
|Institution||UMMS - Graduate School of Biomedical Sciences|
|Department||Molecular Genetics and Microbiology|
Ph. D. (1980) University of California, Berkeley
Cell-Surface Receptors in Saccharomyces cerevisiae
Our laboratory uses the yeast Saccharomyces cerevisiae as amodel system for studying basic problems of hormone receptor functionincluding signal transduction, endocytosis and cellular control ofmembrane protein integrity. These studies focus on the a-factor pheromone receptor. When a-factor pheromonebinds to specific receptors on the surface of yeast cells, it causes thecells to arrest cell division. The occupied receptors are continuallyinternalized and replaced by new receptor synthesis. After prolongedexposure to a-factor, thecells become insensitive to the pheromone and reenter the division cycle. This receptor is a member of the same structural class as the b-adrenergicreceptor, photorhodopsin, and a variety of neurotransmitter receptors,that is, all of these receptors contain seven-transmembrane-spanningdomains and require a heterotrimeric G protein for signal transduction. The projects in our lab utilize both biochemical and genetic techniques togain an understanding of the role that cell-surface receptors play in thegeneration of intracellular signals and the mechanisms by which the cellregulates these signals. Our approach is straightforward. We havedesigned assays that are sensitive to specific functions of the receptorand the G protein (i.e., ligand binding, receptor conformation,receptor endocytosis and the states of G protein aggregation). Mutationsthat are defective for one or more of these functions indicate whether theactivities have physiological relevance.
Membrane protein traffic. Assays for endocytosis in yeast are available that measure the net movement of proteins from the cell surface to the vacuole. However, as yet, no assays have been developed for detecting the recycling of endocytosed proteins back to the cell surface. Although mutants are known that block the net movement of endocytosed proteins to vacuole, it is not known whether they function by blocking the intial internalization step or by increasing the rate at which the endocytosed proteins recycle. Our lab is currently developing an assay that detects recycling of endocytosed membrane proteins. Peptides containing a specific proteolytic cleavage site are attached to proteins on the plasma membrane. Since the enzyme that cleaves this site is present in the internal endocytic compartment, the appearance of cleaved peptides on the plasma membrane indicate that the tagged protein had been recycled. The assay will then be used to test whether known endocytosis mutants block internal movement or accelerate external movement of membrane proteins.
Mutant receptors defective for oligomerization. Although G coupled-protein receptors form dimers and higher-order oligomers in the plasma membrane, the importance of this activity is unknown. We have shown previously that -factor receptors form oligomers. Furthermore, oligomeric receptors are subject to endocytosis since endocytosis-defective mutant receptors become competent for endocytosis when they are coexpressed with wild-type receptors. The object of this project will be to identify mutations in the receptor gene that block endocytosis. A mutant gene that encodes endocytosis-defective receptors (fused to the green fluorescent protein) will be subjected to mutagenesis and then introduced into a strain that produces wild-type receptors. New mutations that block oligomerization will result in failure of the fluorescent receptors to be endocytosed. The DNA sequence of these mutant alleles will indicate the amino acids in the receptor that are required for endocytosis. The mutants will provide a means for testing whether other activities of the receptor (e. g., endocytosis and signal transduction) depend on oligomerization.
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