Direct binding of plectin to Fer kinase and negative regulation of its catalytic activity

Plectin is a cyoskeletal linker protein that protects tissues against mechanical stress. We report here that the N-terminal domain of the nonreceptor tyrosine kinase Fer interacts with N-terminal sequences of plectin. Recombinant protein encoded by exon 12-24 of rat plectin bound directly to amino acid 1-329 of murine Fer. Using an antiserum prepared to a recombinant N-terminal fragment of Fer kinase, plectin was coimmunoprecipitated with Fer from cell lysates of cultured mouse fibroblasts. Plectin was shown to partially colocalize with Fer in these cells. Upon transfection of full length Fer cDNA into plectin-negative mouse fibroblasts, hyperphosphorylation of Fer was observed; hyperphosphorylation was strongly reduced when N-terminal Fer deletion mutants were transfected. Immunocomplex kinase assays showed that the activity of Fer kinase transfected into plectin-negative fibroblasts was increased compared to that transfected into wild type cells. We conclude that Fer interacts with plectin and that this interaction may serve to negatively regulate Fer's activity.