SIGNALING MOLECULES AND THE REGULATION OF INTRACELLULAR-TRANSPORT

A variety of signalling molecules has been implicated over the past 8 years in the regulation of intracellular transport pathways. Those mol ecules include heterotrimeric GTP binding proteins, members of the pro tein kinase C family, and members of the Rho subfamily of small GTPase s. Until recently, no common theme among the three classes of regulato rs was apparent. The finding that all three can influence the activity of phospholipase D (PLD), and the fact that members of the Arf subfam ily of GTPases (with established roles in intracellular transport) are potent activators of PLD suggests the hypothesis that PLD is a focal point for integration of cellular responses to hormone signalling and for membrane homeostasis. Work during the past 2 years is beginning to uncover some transport pathways where PLD involvement is inferred. It is proposed that, if signalling is required to monitor and adjust tra nsport rates to and from the various membrane organelles, the most eco nomical way to achieve this would be to regulate recycling and allow t he concentration of cargo receptors to determine forward transport. (C ) 1998 John Wiley & Sons, Inc.