DEVELOPMENTAL INDUCTION OF GOLGI STRUCTURE AND FUNCTION IN THE PRIMITIVE EUKARYOTE GIARDIA-LAMBLIA

A fundamental characteristic of eukaryotic cells is the presence of me mbrane bound compartments and membrane transport pathways in which the Golgi complex plays a central role in the selective processing, sorti ng, and secretion of proteins, The parasitic protozoan Giardia lamblia belongs to the earliest identified Lineage among eukaryotes and there fore offers unique insight into the progression from primitive to more complex eukaryotic cells, Here, we report that Giardia trophozoites u ndergo a developmental induction of Golgi enzyme activities, which cor relates with the appearance of a morphologically identifiable Golgi co mplex, as they differentiate to cysts. Prior to this induction, no mor phologically or biochemically identifiable Gels complex exists within nonencysting cells. Remarkably, protein secretion in both nonencysting and encysting trophozoites is inhibited by brefeldin A, and brefeldin A-sensitive membrane association of ADP-ribosylation factor and beta- COP is observed, These results suggest that the secretory machinery of Giardia resembles that of higher eukaryotes despite the absence of a Golgi complex in nonencysting trophozoites. These findings have implic ations both for defining the minimal machinery for protein secretion i n eukaryotes and for examining the biogenesis of Golgi structure and f unction.