We have also used rab7 as a molecular handle to identify additional components of the transport machinery and to learn more about the enigmatic late endosome. We have developed chimeras between green fluorescent protein and rab7 to study endosome dynamics. Analysis of such probes in living cells reveals a clear dependence on microtubules for rapid transit of rab7-positive endosomes in both anterograde and retrograde directions (Rab7 Movie, Wild type video). Both genetic and biochemical approaches have allowed us to identify some of the interacting partners of rab7. Thus, a synergistic interaction between rab7 and a phosphatidyl inositol (PI) 3-kinase has been observed that is currently being analyzed in more detail. We have identified a second novel rab GTPase that partially colocalizes with rab7 and work is in progress to identify its transport function and to test for cooperativity with rab7. Using a genetic screen for rab7 interacting proteins two isoforms of a general rab recycling factor called GDI were identified. The in vivo overexpression of either GDI was shown to selectively interfere with the function of rab11 in exocytosis and provided evidence for rab11 as a key regulator of the intersection between endocytic and exocytic pathways (Chen et al., 1998). Together these studies offer strategies for establishing the mechanisms whereby rab proteins and other cofactors regulate intracellular membrane transport on the endocytic and exocytic pathways. More