Insect immunity Inducible antibacterial proteins from Hyalophora cecropia

Sammanfattning: A powerful bactericidal activity can be induced in the hemolymph of many insects as a response to an injection of bacteria. The nature of the effector molecules of this immune response was investigated, using pupae of the Cecropia moth, Hyalophora cecropia. Three major types of antibacterial proteins were found: the cecropins, the P5 proteins, and lysozyme. They appear in the hemolymph as a result of de novo synthesis.Six different cecropins were purified and characterized. The full amino acid sequences of the three major cecropins A, B and D were determined, as well as partial sequences of the three minor cecropins C, E and F. The cecropins are all very small (Mr = 4,000) and basic (pI > 9.5) proteins, and they show extensive homology in their sequences. The three major cecropins are products of different genes. Their C-terminals are blocked by uncharged groups, which can be removed by mild acid hydrolysis. The minor cecropins are closely related to the major forms, and may be unblocked precursors or, in one case (cecropin F), a minor allelic form. The cecropins were shown to be lytic, and to be efficient against several Gram positive and Gram negative bacterial strains, but not against mammalian cells.The P5 proteins are bactericidal proteins, larger than the cecropins (Mr = 20,000 - 23,000). Six forms, differing in isoelectric point, were isolated. They form two closely related groups, the basic (P5 A-D) and the acidic forms (P5 E-F). Within each group, the different forms have almost identical amino acid compositions.The Cecropia lysozyme is similar to lysozymes isolated from other insects, as well as to that from hen egg white. It is lytic to a restricted number of Gram positive bacteria.The presence of cecropins and other antibacterial factors was demon-strated also in other lepidopterans, notably Galleria mellonella, and may explain earlier observations of antibacterial factors in the latter species.