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The physiological role of the GRASP proteins in unconventional secretion and mitotic Golgi fragmentation


The GRASP proteins were identified as factors involved in the stacking of Golgi cisternae in mammalian cells thereby creating a protein regulated mechanism of mitotic Golgi fragmentation. Although these proteins are well conserved, the structure of Golgi membranes and its fragmentation during mitosis is species specific. Phylogenetic analysis of the GRASP proteins compared with the organization of Golgi membrane across eukaryotes suggest these proteins cannot be conserved on the sole basis of Golgi structural proteins. Experimental evidence, in both mammalian tissue culture cells and Dictyostelium discoideum, reveals these proteins do not play a role in Golgi structure or conventional protein secretion; the two biochemical functions initially assigned to them. Surprisingly, the GRASP homolog in Dictyostelium was found to be essential for the unconventional secretion of a protein required during the terminal differentiation of spore cells. This is the first physiologically relevant function found for the GRASP proteins in eukaryotes and suggests a new function of the Golgi membranes on their role in unconventional protein secretion. Furthermore, the mammalian GRASP homolog, GRASP55, is sequentially phosphorylated during mitosis, a necessary step in the fragmentation of Golgi membranes and the entry of cells into mitosis

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