Sequence and specificity of two antibacterial proteins involved in insect immunity

Abstract

Immune responses have been described for many different insect species¹. However, it is generally acknowledged that insects lack lymphocytes and immunoglobulins and their immune systems must therefore differ from those of vertebrates. An effective humoral immune response has been found in pupae of the cecropia moth, Hyalophora cecropia¹. The expression of this multicomponent system requires de novo synthesis of RNA and proteins² and its broad antibacterial activity is due to at least three independent mechanisms³, the most well known of which is the insect lysozyme^4–7. However, this enzyme is bactericidal for only a limited number of Gram-positive bacteria. We recently purified and characterized P9 A and P9B, which are two small, basic proteins with potent antibacteral activity against Escherichia coli and several other Gram-negative bacteria⁷. We believe that P9A and P9B play an important part in the humoral immune responses described previously⁸ and that the P9 proteins represent a new class of antibacterial agents for which we propose the name cecropins. We describe here the primary structures of cecropins A and B. We also show that cecropin A is specific for bacteria in contrast to melittin, the main lytic component in bee venom⁹ which lyses both bacteria and eukaryotic cells.