THE SECRETORY PATHWAY OF PROTISTS - SPATIAL AND FUNCTIONAL-ORGANIZATION AND EVOLUTION

All cells secrete a diversity of macromolecules to modify their enviro nment or to protect themselves. Eukaryotic cells have evolved a comple x secretory pathway consisting of several membrane-bound compartments which contain specific sets of proteins. Experimental work on the secr etory pathway has focused mainly on mammalian cell lines or on yeasts. Now, some general principles of the secretory pathway have become cle ar, and most components of the secretory pathway are conserved between yeast cells and mammalian cells. However, the structure and function of the secretory system in protists have been less extensively studied . In this review, we summarize the current knowledge about the secreto ry pathway of five different groups of protists: Giardia lamblia, one of the earliest lines of eukaryotic evolution, kinetoplastids, the sli me mold Dictyostelium discoideum, and two lineages within the ''crown' ' of eukaryotic cell evolution, the alveolates (ciliates and Plasmodiu m species) and the green algae. Comparison of these systems with the m ammalian and yeast system shows that most elements of the secretory pa thway were presumably present in the earliest eukaryotic organisms. Ho wever; one element of the secetory pathway shows considerable variatio n: the presence of a Golgi stack and the number of cisternae within a stack We suggest that the functional separation of the plasma membrane from the nucleus-endoplasmic reticulum system during evolution requir ed a sorting compartment, which became the Golgi apparatus. Once a Gol gi apparatus was established, it was adapted to the various needs of t he different organisms.