Utilizing genetics and biochemistry to study endosomal function and receptor downregulation in the model organism Saccharomyces cerevisiae. These studies have lead to the identification of a group of gene products whose function is required for the proper function of the multivesicular body (MVB) pathway, critical for receptor downregulation. These mutants are referred to as the class E vacuolar protein sorting mutants, a group comprised of 16 genes. Class E gene products are involved in protein sorting at the endosome, where they are responsible for recognizing and concentrating cargoes destined for entry into the MVB pathway and eventual degradation within the lysosome/vacuole.

Function of the MVB pathway is required for a variety of cellular processes, including lysosome function, receptor downregulation, developmental patterning, immune response and even the budding of certain retroviruses (such as HIV-1) from a host cell. Understanding the molecular mechanisms of MVB sorting is, therefore, important in a number of contexts.

Cells expressing a GFP-tagged form of the ubiquitin ligase Rsp5 (green) and a red endosomal marker were visualized by deconvolving fluorescence microscopy and a three dimensional image was generated.