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Insect hemocytes in response to lipopolysaccharide (LPS) of Gram-negative bacteria facilitate binding and internalization of either cell-associated or cell-free LPS (Charalambidis, N. D., Foukas L. C., and Marmaras V. J. (1996) Eur. J. Biochem.236, 200–206). An early event in LPS signaling in hemocytes involves protein tyrosine phosphorylation (Charalambidis N. D., Zervas C. G., Lambropoulou M., Katsoris P. G., and Marmaras V. J.(1995) Eur. J. Cell Biol.67, 32–41). Here we report further data of LPS-mediated signal transduction responsible for Escherichia coliphagocytosis. We demonstrate that both adhesion of hemocytes to substrata and LPS stimulation can cause activation of p44MAPKin Ceratitis capitatahemocytes but with distinct kinetics indicating different functions. In addition, we showed that Drk, a homolog protein to the mammalian GRB2, is implicated in the transmission of LPS signaling, indicating that the Ras/mitogen-activated protein kinase pathway is involved. Either the cell-free or the cell-associated LPS appears to attach to the hemocyte surface by the same mechanism that is based on the cross-linking of LPS to membrane-associated p47 via the intermediacy of tyrosine derivatives generated by the action of phenol oxidase. By contrast, the cell-free LPS internalization into the hemocytes differs from the cell-associated LPS internalization. For E. coliinternalization integrin receptors as well as cytoskeletal rearrangements are required, as judged by inhibition of E. coliinternalization in the presence of the RGD peptide, β3-integrin antibodies, and cytochalasin D.