Insect immunity. III. Purification and partial characterization of immune protein P5 from hemolymph of Hyalophora cecropia pupae

We previously showed that in pupae of Hyalophora cecropia, eight hemolymph proteins (P1 through P8) were selectively synthetized after immunization (Faye et al., Infect, Immun. 12:1426-1438, 1975). We also showed that a gross fractionation was obtained by a series of ammonium sulfate precipitations (designed A through D) and that protein P5 was enriched in fraction A. Starting from fraction A, we have now purified protein P5 by using dialysis, isoelectric focusing, and hydroxylapatite chromatography. The final product gave a single band in both gradient gel and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using the latter method, proteins P5 and P8 were found to be enriched in fraction A, but they were absent in fraction Ac prepared from nonimmunized pupae. Protein P5 was found to have a pI of 6.6 and a molecular weight of 24,000 in sodium dodecyl sulfate and 96,000 in tris (hydroxymethyl) aminomethane-borate, pH 9.0. These data suggest a structure for P5 composed of four subunits of equal size. Protein P5 stimulated the killing of Escherichia coli by hemolymph fractions B and D, but it had neither killing nor phenol oxidase activity of its own.