Steroid hormones are key regulators of a diverse array of physiological processes, including sodium homeostasis, reproduction, and the development of secondary sex characteristics. These molecules allow tissues to respond in a coordinated manner to changes in the internal and external environments by functioning as ligands for both nuclear and plasma membrane receptors. Because steroid hormones control the expression of numerous genes in virtually all cell types, steroidogenic cells utilize multiple mechanisms that ensure tight control of the synthesis of these molecules. A major goal of our research is to understand the mechanisms by which the pituitary-derived hormone adrenocorticotropin (ACTH) regulates cortisol production by the adrenal cortex. Our research has identified an integral role lipid ligands in regulating the transactivation potential of the nuclear receptor steroidogenic factor-1 (SF-1). Further, we have found that signaling molecules such as ACTH direct temporally distinct and reversible post-translational modifications (PTMs) of several nuclear proteins, including SF-1. These PTMs serve as master regulators of protein function by controlling the ability of modified proteins to participate in varied nuclear processes, including transcription and splicing. We propose that ACTH controls steroid hormone biosynthesis by modulating the PTM of target proteins, thus facilitating the assembly of distinct protein-protein, protein-DNA, and protein-RNA complexes. This research project will test the central hypothesis that activation of the ACTH signaling cascade promotes the PTM of multiple proteins that regulate the transcription and splicing of CYP17. Further, signal-dependent PTMs of SF-1 and coregulatory proteins such as p54nrb, modulate the differential assembly of protein complexes that facilitate the coupling of multiple nuclear processes, including repression, transcriptional initiation, ligand synthesis, transcript elongation and termination, and RNA processing. Specific Aim 1 will determine the mechanism by which p54nrb bridges transcription and splicing by employing mass spectrometric proteomic approaches to analyze of protein complexes and PTMs of p54nrb. These studies will also define how ACTH/cAMP-stimulated PTM regulates the ability of the p54nrb to control varied nuclear processes. Specific Aim 2 will define the mechanism by which p21-activated kinases (PAK) control steroid hormone production in the human adrenal cortex. Substrate trapping and coupled with mass spectrometric, microscopic, and biochemical approaches will be used to identify novel PAK targets and elucidate the functional significance of PAK-catalyzed phosphorylation on protein function and steroidogenic capacity. Specific Aim 3 will define the parameters that control ligand-dependent modulation of SF-1 function. SF-1 is predominantly localized in the nucleus, thus localized nuclear synthesis of lipid ligands is required to promote optimal steroidogenic gene expression. We propose that ACTH/cAMP signaling regulates SF-1 transactivation potential by triggering PTMs that regulate occupancy of the receptor's ligand binding pocket. Mass spectrometric analysis of PTMs and ligands will define the relationship between signal- dependent stabilization of the interactions between SF-1 and ligand and SF-1 and coregulatory proteins.