A 120 kDa Nuclear Phospholipase Cγ1 Protein Fragment Is Stimulated in Vivo by EGF Signal Phosphorylating Nuclear Membrane EGFR

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Abstract
Intraperitoneal injection of epidermal growth factor (EGF) into mice resulted in the phosphorylation of liver nuclei phospholipase Cγ1 (PLCγ1) at the tyrosine, coincident with the time course of nuclear membrane epidermal growth factor receptor (EGFR) activation. The function of PLCγ1 in mice liver nuclei was attributed to a 120 kDa protein fragment. This 120 kDa protein was immunoprecipitated with the isozyme specific PLCγ1 antibody and was found to be sensitive to a PLCγ1 specific blocking peptide. The 10-partial sequence analysis revealed that the 120 kDa protein contains the PELCQVSLSE sequence at its N-terminal end and the RTRVNGDNRL sequence at its C-terminal end, which reveals that this protein is a major fragment of PLCγ1 devoid of an amino acid portion at the N-terminal end. The tyrosine-phosphorylated 120 kDa protein interacts with activated EGFR, binds phosphatidylinositol-3-OH-kinase enhancer (PIKE), enhances nuclear phosphatidylinositol-3-OH-kinase (PI[3]K) activity, and generates diacylglycerol (DAG) in response to the EGF signal to the nucleus in vivo. The immunoprecipitated 120 kDa protein fragment displayed phosphatidylinositol (PI) hydrolysis activity. These results establish the capacity of EGF-triggered nuclear signaling which is mediated by EGFR itself, located on the inner nuclear membrane. This is the first report identifying a 120 kDa PLCγ1 fragment generated in vivo in the nucleus and capable of discharging the function of nuclear PLCγ1.