MVB Cargo Recognition
The plasma membrane of a typical cell contains a large number of receptors that perceive extracellular signals and induce the appropriate intracellular signaling cascades. An important mechanism by which a cell can modulate the duration and intensity of these signals is by targeting activated receptors to the lysosome for degradation, or "downregulation". Inability to appropriately downregulate activated receptors, such as the epidermal growth factor receptor (EGFR), results in increased steady-state levels of activated receptors and prolonged signaling through downstream cascades, contributing to both cell transformation and defects in developmental patterning. Downregulation of EGFR is tightly controlled and ultimately mediated by the multivesicular body (MVB) sorting pathway. During MVB formation, cargoes destined for delivery to the hydrolytic lumen of the lysosome/vacuole (such as activated EGFR) are actively sorted into vesicles that invaginate and bud into the lumen of the endosome; giving rise to the characteristic multivesicular morphology. Heterotypic fusion of the MVB with the lysosome/vacuole results in the delivery of these intralumenal vesicles (containing cargo proteins) to the hydrolytic lumen of the lysosome for degradation (see Figure 1, below.)
Function of the MVB pathway is critical for a number of cellular phenomena. Inability to appropriately downregulate the epithelial sodium channel confers an inherited form of hypertension (Liddle's syndrome). Major Histocompatability Complex II (MHCII)-mediated immune response relies upon the function of an MVB-like structure, the MHCII-compartment. Aberrant trafficking of lipids through this pathway also contributes to a number of human disease states, including atherosclerosis. Finally, the machinery that drives MVB sorting is usurped by certain viruses during their life cycle (e.g. HIV-1). We have chosen the model eukaryotic organism Saccharomyces cerevisiae to study the mechanism of cargo identification and sorting into the MVB pathway. We have utilized a combination of genetics and biochemistry to identify both cis- and trans-acting components of the MVB sorting pathway. These studies revealed that ubiquitin serves as a cis-acting sorting determinant for inclusion into the MVB pathway, and also identified the ESCRT machinery as requisite trans-acting machinery involved in this process. The ESCRT proteins are conserved in higher eukaryotes and their dysfunction has been linked to defects in organismal development, as well as tumor susceptibility. We are presently focused on several aspects of MVB pathway, including cargo identification and recognition. Defining the molecular mechanisms that govern MVB cargo identification and sorting will yield significant contributions to the understanding of protein and lipid sorting in normal and disease states.
Figure 1 - Model for MVB sorting