DESCRIPTION (provided by applicant): The p90 Ribosomal S6 kinases (RSKs) are immediately downstream effectors of mitogen activated protein kinases and play a major role in regulation of cell proliferation and survival. Among the four isoforms, RSK4 is the most dissimilar and also functionally different from the other isoforms. The upregulation of RSK1 and RSK2 also predisposes cells to transformation and tumor formation. Moreover, in the heart, RSK1 is involved regulating pathophysiological processes such as hypertrophy. This application is based on our recent findings that inactive RSK1 interacts with the regulatory subunit (Rl) of cAMP dependent protein kinase (PKA) while the phosphorylated, active RSK1 interacts with the catalytic subunit of PKA (PKAc). The association of inactive RSK1 with Rl decreases interactions between PKAc and Rl. In contrast, the association of phospho- RSK1 with PKAc increases interactions between PKAc and Rl and decreases the ability of cAMP to active the PKA holoenzyme. Additionally, we have shown that the interactions of inactive RSK1 and active RSK1 with subunits of PKA permits the RSK1 to exist in a complex with PKA anchoring proteins (AKAPs) and the disruption of PKA interactions with AKAPs dramatically alters the distribution of active RSK1 in cells. Thus, when interactions of PKA with AKAPs are intact, the active RSK1 is localized primarily in the nucleus of cells. On the other hand, when the PKA/AKAP interactions are abolished, the amount of active RSK1 in the nucleus is decreased and its amount in the cytoplasm is increased with a resultant increase in phosphorylation of the cytosolic RSK1 substrates tuberous sclerosis complex 2 (TSC2) and BAD. Increased phosphorylation of BAD by RSK1 is associated with an increase in protection from cellular apoptosis. Given these findings, our central hypothesis is that the interactions of RSK1 with PKA and AKAPs are of functional significance in regulating the activity of PKA as well as modulating the subcellular localization of RSK1 and its biological actions. To address this hypothesis and to identify the mechanisms that regulate interactions between RSK1 and PKA subunits, we will pursue the following specific aims. Aim 1: To identify the regions on RSK1 and the subunits of PKA that interact with each other. Aim 2: To elucidate the mechanisms involved in regulation of PKA by RSK1 and to determine the regulation of RSK1/PKA subunit interactions. Aim 3: To determine the role of RSK1/PKA subunit interactions in the cellular distribution of RSK1, the activation of RSK1, and regulation of its biological functions. These aims will identify novel mechanisms by which PKA activity is regulated and also elucidate of the role of the interactions between RSK1 with PKA subunits or AKAPs in regulating the biological actions of RSK1. These novel insights may then permit the development of specific interventions that regulate certain functions of both these kinases.