Calmodulin-dependent protein kinases phosphorylate gp130 at the serine-based dileucine internalization motif

The receptor for leukemia inhibitory factor (LIF) consists of two polypeptides, the low affinity LIF receptor (LIFR) and gp130. We previously demonstrated that LIF stimulation caused phosphorylation of gp130 at Ser782, adjacent to a dileucine internalization motif, and that transient expression of a mutant receptor lacking Ser782 resulted in increased cell surface expression and increased LIF-stimulated gene expression compared to wild-type receptor. Phosphorylation of Ser782 on gp130 fusion protein by LIF-stimulated 3T3-L1 cell extracts was inhibited 61% by autocamtide-2-related inhibitory peptide (AIP), a highly specific and highly effective inhibitor of calmodulin-dependent protein kinase type II (CaMKII). Purified rat forebrain CaMKII was also able to phosphorylate gp130 fusion protein at Ser782 in vitro. Furthermore, antibodies targeting CaMKII and CaMKIV were able to immunoprecipitate gp130 phosphorylating activity from LIF-stimulated 3T3-L1 lysates. While pretreatment of cells with the MAPKK inhibitors PD98059 and U0126 blocked phosphorylation of Ser782 prior to LIF stimulation, these inhibitors did not block Ser782 phosphorylation by LIF-stimulated 3T3-L1 cell extracts in vitro. These results show that CaMKII and possibly CaMKIV phosphorylate Ser782 in the serine-based dileucine internalization motif of gp130 via a MAPK-dependent pathway.