The GM130 and GRASP65 Golgi proteins cycle through and define a subdomain of the intermediate compartment

Integrating the pleomorphic membranes of the intermediate compartment (IC) into the array of Golgi cisternae is a crucial step in membrane transport, but it is poorly understood. To gain insight into this step, we investigated the dynamics by which cis-Golgi matrix proteins such as GM130 and GRASP65 associate with, and incorporate, incoming IC elements. We found that GM130 and GRASP65 cycle via membranous tubules between the Golgi complex and a constellation of mobile structures that we call late IC stations. These stations are intermediate between the IC and the cis-Golgi in terms of composition, and they receive cargo from earlier IC elements and deliver it to the Golgi complex. Late IC elements are transient in nature and sensitive to fixatives; they are seen in only a fraction of fixed cells, whereas they are always visible in living cells. Finally, late IC stations undergo homotypic fusion and establish tubular connections between themselves and the Golgi. Overall, these features indicate that late IC stations mediate the transition between IC elements and the cis-Golgi face.