Ubiquitin Mediated Trafficking to the Lysosomal Degradative Pathway

Our laboratory is interested in how proteins are degraded in lysosomes. This is a fundamental process of all eukaryotic cells necessary for regulating a variety of cell surface proteins. This process is often termed "downregulation", and is a central feature of virtually all physiological processes that rely on cell surface membrane proteins. Failure to properly downregulate particular proteins can lead to or exacerbate a variety of pathophysiological conditions such as cancer, hypertension, and cardiac disease. Our overall goal is to understand the protein machinery common in all cell types that controls the delivery of membrane proteins to the lysosome. There are two processes that we examine: the first is how proteins are designated and recognized for delivery to the lysosome. The second is how transport through the endocytic pathway via endosome membrane fusion events is controlled. Our primary focus is one finding out how individual "cargo" proteins are selected and designated for transport and degradation in the lysosome. Membrane proteins can initiate their journey to lysosomes from a number of cellular compartments. At the cell surface, proteins can enter the endocytic pathway via internalization. At the Golgi apparatus, proteins can be sorted into transport vesicles that are targeted to endosomes. Perhaps the most critical sorting step controlling the degradation of membrane proteins in lysosomes, is the incorporation of protein cargo into vesicles that bud into the lumen of the endosome. This sorting step occurs within endosomes and leads to the formation of multivesiculated bodies (MVBs) that accumulate lumenal membranes. These lumenal membranes are then subject to degradation by lysosomal lipases and proteases, thereby ensuring the complete destruction of integral membrane proteins. A variety of studies in both yeast and mammalian cells have established that all of these sorting steps can be controlled by the post-translational attachment of ubiquitin. Ubiquitin is a 76 amino acid peptide that is covalently linked to lysine residues via an isopeptide bond. When attached to membrane proteins, ubiquitin acts as a sorting signal to incorporate cargo into transport vesicles that ultimately leads to their delivery to lysosomes. Currently we are determining how ubiquitinated cargo is recognized and incorporated into transport vesicles destined for the lysosome.