Nature of the anchors of membrane-bound aminopeptidase, amylase, and trypsin and secretory mechanisms in Spodoptera frugiperda (Lepidoptera) midgut cells

Spodoptera frugiperda larvae have a microvillar aminopeptidase and both sol uble and membrane-bound forms of amylase and trypsin. Membrane-bound aminop eptidase is solubilized by glycosyl phosphatidylinositol-specific phospholi pase C (GPI-PLC) and detergents, suggesting it has a GPI anchor. Membrane-b ound trypsin is not affected by GPI-PLC, although it is solubilized by papa in and by different detergents. Membrane-bound amylase is similar to trypsi n, although once solubilized in detergent it behaves as a hydrophilic prote in. Musca domestica trypsin antiserum cross-reacts with only one polypeptid e from S. frugiperda midgut. With this antiserum, trypsin was immunolocaliz ed in the anterior midgut cells at the microvillar surface and on the membr anes of secretory vesicles found in the apical cytoplasm and inside the mic rovilli. The data suggest that in this region trypsin is bound to the secre tory vesicle membrane by a hydrophobic anchor. Vesicles migrate through the microvilli and are discharged into the lumen by a pinching-off process. Tr ypsin is then partly processed to a soluble form and partly, still bound to vesicle membranes, incorporated into the peritrophic membrane. In posterio r mid,out cells, trypsin immunolabelling is randomly distributed inside the secretory vesicles and at the microvilli surface, suggesting exocytosis. A mylase probably follows a route similar to that described for trypsin in an terior midgut, although membrane-bound forms (peptide anchor) solubilize ap parently as a consequence of a pH increase inside the vesicles. (C) 1998 El sevier Science Ltd. All rights reserved.