2 Protein Secretion, Membrane Biogenesis, and Endocytosis

Abstract

I. INTRODUCTION
A growing yeast cell doubles its surface area every division cycle. The principal function of the yeast secretory pathway is to generate and deliver new membrane and protein to the growing surface of the bud. The pathway that newly synthesized proteins follow to the cell surface has the same outline in Saccharomyces cerevisiae as that originally defined in mammalian cells (Palade 1975). The principal steps in the pathway are outlined below and are diagrammed in Figure 1. The gene products responsible for the function of the secretory pathway are given at the end of this chapter in Table 1.

Secretory proteins are translocated from the cytosol into the lumen of the endoplasmic reticulum (ER). For some proteins, translocation occurs cotranslationally and for other proteins, translocation can occur posttranslationally, i.e., after completion of protein synthesis.

Secretory proteins assume a functional conformation within the lumen of the ER. This can involve covalent modifications of the protein such as cleavage of signal peptide, addition of N-linked and O-linked carbohydrate chains, disulfide bond formation, and addition of glycosylphosphatidylinositol (GPI) anchors. Catalyzed protein folding and, in some cases, assembly of multimeric protein complexes also take place in the ER lumen.

Secretory proteins are then packaged into 60-nm vesicles that bud from the ER.

These vesicles then deliver their contents to the Golgi by fusion with the Golgi membrane.

The Golgi apparatus is composed of three biochemically distinguishable compartments. Secretory proteins are carried through successive compartments of the Golgi...