The mammalian target of rapamycin, mTOR, plays a key role in the control of a number of cell functions. The best-understood of these is protein synthesis, where mTOR regulates proteins involved in the initiation and elongation stages of mRNA translation. These proteins are regulated via changes in their states of phosphorylation. Although mTOR itself has protein kinase activity, it is clear that mTOR does not itself catalyze all these phosphorylation events. mTOR forms complexes with several partner proteins to yield complexes termed mTOR complexes 1 and 2. Rapamycin inhibits many of the functions of mTORC1, but not mTORC2. One component of mTORC1 is raptor, which acts as a scaffold protein by interacting with proteins that are substrates for phosphorylation by mTOR via their TOR-signalling (TOS) motifs. Signalling through mTORC1 is activated by amino acids and by anabolic or mitogenic stimuli (e.g., insulin, growth factors). The mechanism by which amino acids stimulate mTORC1 is unclear, and unanswered questions remain about its activation by hormones and growth factors. The latter is known to involve the small G-protein Rheb, which acts to stimulate signalling through mTORC1. It is clear that mTORC1 signalling is important for cell proliferation and cell growth. The latter is exemplified by the fact that rapamycin can prevent, or even reverse, cardiac hypertrophy, a condition that is characterized by elevated rates of protein synthesis and increased size of cardiomyocytes. To test the role of Rheb and mTORC1 signaling in regulating protein synthesis in heart cells, we expressed Rheb in adult rat cardiomyocytes using an adenoviral vector. This increased the rate of protein synthesis and led to an increase in cell area and cell volume. Both effects were inhibited by rapamycin. This demonstrates that Rheb-induced activation of mTORC1-signalling is sufficient to stimulate the growth of cardiomyocytes. We are currently examining which mTORC1-activated steps in mRNA translation are important for the activation of protein synthesis in cardiomyocytes. In yeast, 14-3-3 proteins play a key role in rapamycin-sensitive signalling. These proteins bind to phosphorylated proteins and may modify their functions in a variety of ways. PRAS40 (proline-rich Akt substrate, 40kDa) can interact with 14-3-3 proteins, and this requires both amino acids and insulin, a feature that suggested it might be regulated by mTORC1. Overexpression of Rheb overcomes the requirement for amino acids, consistent with the idea that PRAS40 is indeed controlled by mTORC1. We have found that PRAS40 interacts with raptor and that this requires a TOS motif within PRAS40. In fact, PRAS40 associated stably with raptor/mTOR, indicating that it is a new component of mTORC1. PRAS40 is phosphorylated by mTOR in vitro, and this occurs at a novel site, distinct from the one phosphorylated by the insulin-stimulated kinase Akt (protein kinase B). To test the cellular role of PRAS40, we used siRNAs to knock down its expression. Surprisingly, this impaired signalling from mTORC1 to targets such as ribosomal protein S6. Thus, PRAS40 is a novel component of, and target for, mTORC1. As mentioned above, mTORC1 controls the elongation phase of mRNA translation. It does so by regulating the kinase that phosphorylates and inactivates eukaryotic elongation factor 2 (eEF2). eEF2 mediates the movement of the ribosome along the mRNA. Signaling downstream of mTORC1 brings about the inactivation of eEF2 kinase and thus the dephosphorylation/activation of eEF2. This regulation of eEF2 kinase involves its mTORC1-dependent phosphorylation at 3 or more sites (Ser78/359/366). However, eEF2 kinase does not possess a TOS-motif and is not a substrate for mTORC1. This indicates that additional kinase(s) link mTORC1 to the control of eEF2 kinase. We therefore developed assays to allow us to screen for kinases acting at regulatory sites in eEF2 kinase. Using this, we have purified and identified the kinase that phosphorylates Ser359 in eEF2 kinase. This enzyme is a cyclin-dependent kinase, whose activity is positively regulated by amino acids. Judging by several criteria, it is a target for control by mTORC1 signalling. Further data on its regulation will be presented. In addition to providing one link between mTOR signalling and the control of eEF2 kinase, this discovery may have important implication for the control of the cell cycle by mTORC1. Such links are of particular interest given that hyperactivation of mTORC1 leads to dysregulation of cell proliferation and to cancer.