The role of arylphorin, an insect storage hexamer, in tobacco hornworm, Manduca sexta.

Abstract

The hypothesis that Manduca sexta arylphorin is a storage protein that provide amino acids for adult protein synthesis was proposed based on its development profile. This study provided experimental evidence supporting this hypothesis. First, preparation of radiolabeled arylphorin of Manduca sexta was compared by in vivo and in vitro biosynthesis from ¹⁴C-phenylalanine. The latter procedure proved to be optimal for obtaining high specific activity. Then turnover of amino acid during adult development was analyzed for phenylalanine and alanine, which represent general patterns of amino acid utilization because of their different metabolic pathways. The pool size and turnover rate of phenylalanine and alanine differed but a similar pattern of changes occurred in rate, being maximum in early and late adult development while at a minimum during middle stage. The pool size was fairly constant with a slight decrease toward the end of development. The absolute values for alanine were consistently higher, varying between 2- to 3-fold higher for pool sizes and 4- to 8-fold for turnover rates. Finally the fate of labeled phenylalanine from free amino acid pool and from arylphorin was compared by injecting ¹⁴C-phenylalanine or ¹⁴C-phe-arylphorin into pupae. The proportion of radioactivity distributed into most tissues or organs of pupae injected with the different labels during either middle or late adult development appeared to have no significant difference. It is suggested that arylphorin is hydrolyzed to free amino acids that mix with the free amino acid pool and are then used for adult development. A small difference in percentage of radioactivity occurred in some organs (i.e., thorax, and leg and wing) indicating that a small amount of arylphorin may be used as intact protein for cuticle formation. The hypothesis that Manduca sexta arylphorin is a storage form for replenishing the free amino acid pool is now confirmed by experimental evidence.