Elk-1 is a novel protein-binding partner for FAK, regulating phagocytosis in medfly hemocytes

Focal adhesion kinase (FAK) and its downstream signaling targets, mitogen-activated protein kinase (MAPKs), are implicated in the process of phagocytosis by insect hemocytes. The goal of this study was to explore further the signaling pathways underlining the process of phagocytosis. The combination of bioinformatics, biochemical, and immunofluorescence approaches strongly support the expression of Elk-1-like protein in medfly hemocytes. Elk-1 is phosphorylated in E. coli or latex beads-challenged hemocytes and osmotic loading experiments as well as flow cytometry analysis demonstrated that Elk-1-like protein regulates the uptake of bacteria. RNA interference (RNAi) and pharmacological inhibitors show that the signaling for Elk-1 phosphorylation is transmitted via FAK/Src and MAPKs pathways. Furthermore, confocal analysis clearly shows that FAK and the phosphorylated FAK at Y397 are localized in the nucleus and cytoplasm, whereas, the phosphorylated Elk-1-like protein is exclusively localized in the nucleus. Finally, co-immunoprecipitation and reciprocal co-immunoprecipitation analysis demonstrated the association of low molecular weight protein bands recognized by FAK antibodies, with Elk-1 or phospho-Elk-1 at ser 383 and confocal microscopy specifies that this association occurs only in the nucleus. These results are strongly supporting that Elk-1-like protein is a novel protein-binding partner for FAK, a finding that significantly broadens the potential functioning of FAK and Elk-1 generally. Evidently, the complex participates in the process of phagocytosis in medfly hemocytes.