The cytoplasmic environment of eukaryotic cells is subdivided into a number of functionally distinct membrane-enclosed organelles. The accurate and efficient delivery of proteins to specific intracellular organelles is essential to establish and maintain the functional integrity of these compartments. The secretory pathway of eukaryotic cells is responsible for the proper modification and delivery of proteins to the cell surface and to a variety of intracellular destinations (Palade 1975). The delivery of proteins to the lysosomal, or vacuolar, compartment is mediated by the secretory pathway and is one of the best-characterized examples of an intracellular protein-sorting process (Kornfeld and Mellman 1989; Klionsky et al. 1990; Raymond et al. 1992; Stack et al. 1995a). Following translocation across the endoplasmic reticulum, lysosomal proteins transit to the Golgi complex together with proteins destined for secretion from the cell. Within a late Golgi compartment, lysosomal proteins are sorted away from the secretory protein traffic and...