Covalent association of lipopolysaccharide at the hemocyte surface of insects is an initial step for its internalization--Protein-tyrosine phosphorylation requirement.

It is well known that lipopolysaccharide (LPS) of Gram-negative bacteria triggers antibacterial responses to mammalian macrophages [Weinstein, S., Gold, M. R. & DeFranco, A. (1991) Proc. Natl Acad. Sci. USA 88, 4148-4152] and insect hemocytes [Charalambidis, N.D., Zervas, C.G., Lambropoulou, M., Katsoris, P.G. & Marmaras, V.J. (1995) Eur J. Cell Biol. 67, 32-41], via protein-tyrosine phosphorylation. In this study we show that insect hemocytes in response to LPS facilitate internalization of LPS (either cell-associated or cell-free). According to our data, the recognition and covalent association of LPS (either cell-associated or cell-free) to the hemocyte surface are essential initial steps for LPS internalization. LPS (Escherichia coli) recognizes membrane effector 47-kDa protein (p47) and then crosslinks to membrane-associated p47 (mp47) via the intermediacy of tyrosine derivatives generated by the action of phenol oxidase, as is the case for cuticular protein-chitin crosslinks during sclerotization [Shaefer, J., Kramer, K.J., Garbow, J.R., Jacob, G.S., Stejskal, E.O., Hopkins, T.L. & Speirs, R.D. (1987) Science 235, 1200-1204]. The covalent association of LPS to the hemocyte surface appears to be a prerequisite for LPS internalization as judged by the resistance of LPS binding to dissociation by proteinase K. In addition, our results show that the effector molecules participating in LPS covalent association at the cell surface and LPS internalization are not involved in LPS-induced activation of hemocytes. However, the fact that genistein, as well as the inhibitors of LPS-dependent secretion, block LPS covalent association at the cell surface and LPS internalization provides a preliminary characterization of an LPS signal-transduction-dependent process which is apparently involved.