Cell proliferation is regulated by extracellular mitogens via two distinct types of signal transduction pathways: one transduces the signals from the membrane to the nucleus, resulting in transcriptional regulation; the other involves membrane-to-cytoplasm signalling that leads to translational regulation. Whereas the membrane-to-nucleus pathways have been extensively studied, as exemplified by the well-known ras/MAP kinase cascade, pathways leading to translational regulation are poorly characterized at the molecular level. The long-term goal of this research is to understand the molecular mechanism of signal transduction leading to translational regulation. A working hypothesis for the current research is that the human protein FRAP (FKBP 12-rapamycin-associated protein), a member of the recently discovered family of phosphatidylinositol kinase (PIK)-related kinases, plays a major role in the rapamycin-sensitive signalling pathway that controls translational initiation. This proposal focuses on exploring the biochemical mechanism of FRAP's function and delineating the FRAP signalling pathway. In the system of mammalian tissue culture cells, a biochemical approach combined with molecular biology and genetic methodologies will be employed to address specific questions such as: What is the function of the FKBP12-rapamycin-binding domain in FRAP and what is the mechanism for rapamycin inhibition of this pathway? What is the physiological substrate for the FRAP kinase? How does FRAP propagate signals to downstream effectors? What are the other components of the pathway and how do they interact to create the circuitry of signal transduction in this pathway? Understanding the biochemistry and regulation of the FRAP protein will not only be essential for unraveling its cellular function, but may also provide insight into the mechanisms of action for other members of the PIK-related kinase family. Delineating the FRAP pathway should ultimately lead to answers to the fundamental question of how the cell cycle machinery is coupled to the translational machinery to achieve regulation of cell proliferation at the translational level.