The membrane transport tartar p115 recycles only between homologous compartments in intact heterokaryons

Cytosolic proteins that participate in membrane traffic are assumed to be r ecruited from the cytosol onto specific membrane sites where they perform t heir function, and then released into cytosol before rebinding to catalyze another round of transport. To examine whether the ER to Golgi transport fa ctor p115 recycles through release into a cytosolic pool, we formed heterok aryons between rat NRK and simian COS-7 cells and examined the dynamics of rat p115 transfer from the rat to the simian portion of the heterokaryon, T he heterokaryons shared a common cytosolic pool, as shown by the efficient relocation of a cytosolic green fluorescent protein (GFP) from the COS-7 to the NRK part of the heterokaryon. Unexpectedly, even 24 h after cell fusio n, rat p115 did not redistribute to the COS-7 part of the heterokaryon. Thi s was not due to the inability of the rat p115 to associate with simian mem branes since rat p115 expressed in COS-7 cells was efficiently targeted to and associated with simian Golgi complex. Furthermore, rat p115 associated with heterologous simian membranes after the NRK and COS-7 Golgi fused into a single chimeric structure. Our results indicate that p115 is not freely diffusible in intact cells and might remain tethered to membranes throughou t its life cycle. These findings suggest that p115, and perhaps other cytos olic proteins involved in membrane traffic, recycle not by being released i nto cytosol, but in association with recycling membranes.