The goal of this proposal is to determine the molecular events that regulate the cell surface expression of the receptors for the neurally active cytokine, leukemia inhibitory factor (LIF). The receptor for LIF consists of two polypeptides, the low affinity LIF receptor (LIFR) and gp130. Recently, it was shown that the primary site of serine phosphorylation on gp130 in response to LIF activation is 5760, immediately preceding a di-leucine motif important for receptor internalization. In addition, a GCSFR/gp130 chimeric receptor lacking 5760 was expressed at 2-10-fold higher levels in Cos-7 cells, suggesting that 5760 is important for regulating the surface expression of gp130. Interestingly, Dittrich et al. showed that replacing 5758 with alanine attenuated rapid endocytosis of gp130, suggesting that this serine is also important in the regulation of gp130. Thus, this proposal has four specific aims: i) to determine whether 5758 is also phosphorylated on gp130, ii) to determine the effects of removing 5758 and 5760 on the phosphorylation, cell surface expression and agonist- mediated internalization of gp130 and LIFR using transiently transfected epitope-tagged chimeric receptor constructs in Cos-7 cells, iii) to establish the functional significance of these phosphorylation events in signalling of the LIF receptor in SN56 neuronal cells and iv) to further characterize the kinase that phosphorylates gp130 on 5760 by testing its sensitivity to various kinase specific inhibitors. The proposed experiments will increase our understanding of the molecular mechanisms that modulate downregulation the gp130/LIFR complex, and will contribute to our understanding of the motifs important for agonist-mediated receptor endocytosis in general.