Identification of Proteins Required for Ca²⁺-Triggered Secretion

Abstract

Eukaryotic cells synthesize proteins whose final destinations are the extracellular space, the plasma membrane or intracellular organelles. The constitutive secretory pathway employed to sort proteins consists of a series of inter-organelle transport steps that are vesicle-mediated. The cellular mechanisms employed in this pathway are under active investigation since such studies promise to reveal insights into the poorly understood biochemical events involved in vesicle budding, vesicle targeting and membrane fusion. Individual vesicle transport steps of the constitutive pathway from the endoplasmic reticulum to plasma membrane fusion have been successfully reconstituted in semi-intact cells or in homogenates.¹ These in vitro reactions typically exhibit a requirement for ATP and cytosolic proteins², and several cytosolic proteins have been purified and characterized, including the 76 kD NSF protein, 35–39 kD SNAP proteins, a 25 kD POP protein, low molecular weight GTP-binding proteins, and components of a non-clathrin coat.^3,4 Genetic approaches in Saccharomyces cerevisiae have complemented the in vitro reconstitution approach, and a large number of genes required in the yeast secretory pathway have been identified by mutational analysis.⁵ Although the biochemical events underlying membrane fusion have eluded detailed description thus far, the identification of several key protein participants represents an important step toward this goal.