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 investigate
d 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 c
onstellation of mobile structures that we call late IC stations. These stat
ions are intermediate between the IC and the cis-Golgi in terms of composit
ion, 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 fi
xatives; 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 transit
ion between IC elements and the cis-Golgi face.