Further, whereas molecular studies have shown that LHX3 regulates nervous system genes within multiprotein complexes and that the N-terminal LIM domains and HD likely mediate these interactions, pituitary gene control requires the actions of additional LHX3 protein regions, including the critical C-terminal activation and repression domains. Together, these in vitro and in vivo observations are consistent with the hypothesis that the nervous system functions of LHX3 are molecularly separable in that the C-terminal part of the protein is only essential for full implementation of the pituitary roles of the protein. LHX3 proteins trans-activate pituitary hormone genes, such as alpha glycoprotein subunit, the TSH beta subunit, prolactin and other pituitary-expressed genes. LHX3 has been shown to interact with other nuclear and regulatory proteins, such as NLI/LDB, ISL1/2, PIT1, RLIM, SLB, MRG1 and CREB Binding Protein. However, the role of the C-terminus in LHX3- mediated pituitary gene activation is not understood and the partners through which it exerts it functions have not been identified. In order to further understand the role of the LHX3 C-terminus in pituitary gene transcriptional regulation, we performed an affinity purification screen to identify proteins interacting with this important region of the protein. This approach identified company website interactions with components of the Inhibitor of histone acetyltransferase multi-subunit complex. INHAT is a multifunctional repressor that inhibits histone acetylation and modulates chromatin structure. The leucine-rich acidic nuclear protein, LANP and template activating factor 1b subunits of INHAT interact with the C-terminus of LHX3 and changes in LANP and TAF-1b levels modulate LHX3-mediated pituitary gene activation. LANP and TAF-1b are two main subunits of the INHAT multisubunit complex that acts as a multifunctional repressor to inhibit histone acetylation and modulate chromatin structure. In a mechanism known as ‘‘histone masking’’, INHAT binds to histone tails to prevent the substrate from interacting with histone acetyltransferases. LANP has been implicated in other processes, including neuronal differentiation, RNA shuttling, microtubule-based functions, apoptosis and inhibition of protein phosphatase 2A. The other principal INHAT subunit, TAF-1b, also has been associated with various functions, such as inhibition of phosphatase PP2A, apoptosis and cell cycle regulation. TAF-1 has been shown to be a linker histone chaperone protein involved in histone H1 dynamics. The INHAT complex also modulates nuclear hormone receptor function. For example, TAF-1b and LANP repress p300-mediated acetylation of estrogen receptor a thereby inhibiting ERamediated transcription. INHAT also interacts with the progesterone receptor and the thyroid hormone receptor beta, suggesting a role in repressing other nuclear receptor-mediated transcription. Similarly, TAF-1b interacts with the glucocorticoid receptor DNA binding domain to suppress GR-induced transcriptional activity. In accord with our findings of INHAT interaction with LHX3, the observations with nuclear receptor transcription factors suggest that INHAT plays diverse and important roles in regulating gene activation. Previous data from our laboratory suggested that the C-terminal domain of LHX3 was important for overall LHX3 activity. The C1 domain has a nuclear localization signal, which functions in a combinatorial fashion with other NLSs to shuttle LHX3 to the nucleus, as well as conserved residues that are targets of kinases. The C2 domain contains the major trans-activation domain of LHX3 which is important for the function of the M2-LHX3 isoform, and the C3 domain, whose function is not yet described, is the most conserved domain of the LHX3 C-terminus across species. Interaction of the LHX3 C-terminus with INHAT may modify activity of LHX3; most notably, our data point to a role in modulating the transcriptional activity of LHX3. Recently, a subset of patients with CPHD was found to have an LHX3 protein lacking the C-terminus, resulting from an early termination signal at residue 224. These patients present with less severe neuroendocrine symptoms than other known human LHX3 mutations, including delayed onset of CPHD, apparently normal pituitary morphology and a lack of the characteristic rigid neck phenotype. Our lab recently developed and characterized a mouse model of the W224Ter patients, demonstrating that the C-terminus is necessary for pituitary development, but not nervous system development. Therefore, understanding the proteins interacting with the C-terminus provide novel insight into the role of this domain in pituitary gene activation and maintenance.