Capacity of the Golgi Apparatus for Cargo Transport Prior to Complete Assembly

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Abstract
In yeast, particular emphasis has been given to endoplasmic reticulum (ER)-derived, cisternal maturation models of Golgi assembly while in mammalian cells more emphasis has been given to golgins as a potentially stable assembly framework. In the case of de novo Golgi formation from the ER after brefeldin A/H89 washout in HeLa cells, we found that scattered, golgin-enriched, structures formed early and contained golgins including giantin, ranging across the entire cis to trans spectrum of the Golgi apparatus. These structures were incompetent in VSV-G cargo transport. Second, we compared Golgi competence in cargo transport to the kinetics of addition of various glycosyltransferases and glycosidases into nascent, golgin-enriched structures after drug washout. Enzyme accumulation was sequential with trans and then medial glycosyltransferases/glycosidases found in the scattered, nascent Golgi. Involvement in cargo transport preceded full accumulation of enzymes or GPP130 into nascent Golgi. Third, during mitosis, we found that the formation of a golgin-positive acceptor compartment in early telophase preceded the accumulation of a Golgi glycosyltransferase in nascent Golgi structures. We conclude that during mammalian Golgi assembly components fit into a dynamic, first-formed, multigolgin-enriched framework that is initially cargo transport incompetent. Resumption of cargo transport precedes full Golgi assembly.