Efficient secretion of attacin from insect fat-body cells requires proper processing of the prosequence

The attacins constitute a group of immune proteins induced after bacterial infection in the moth Hyalophora cecropia. They are synthesized as preproproteins and undergo post-translational modification during transport to the hemolymph. The processing and transport rates of attacin were studied in its natural host as a response to infection. Monensin totally inhibited the processing from proattacin to attacin and the radiolabeled proattacin remained intracellular. This observation indicated that the prosequence is removed at or after the trans-Golgi compartment. It is also suggested that the processing of the prosequence does not occur in acidic vesicles, as the process was not inhibited by the weak base chloroquine. To study prosequence function, the attacin gene and genes with mutations in the prosequence were cloned into the baculovirus Autographa californica nuclear polyhedrosis virus. The processing of proattacin and the transport of attacin were studied by pulse-chase experiments with fat body isolated from Trichoplusia ni larvae. The rate of secretion from fat body was lowest for proattacin, which could not be processed to attacin, intermediate for attacins lacking the prosequence and highest for natural attacin. We could not detect any biological activity for proattacin.