Detergent-resistant domains in Spodoptera frugiperda midgut microvillar membranes and their relation to microapocrine secretion.
The midgut from lepidopteran insects has a particular way to release proteins to the lumen, named microapocrine secretion that could be an adaptation to release secretory contents into the lumen at water absorbing regions. In this process small vesicles (microapocrine vesicles) bud from the midgut microvilli as double membrane vesicles, where the inner membrane comes from the secretion vesicle and the outer one from the microvillar membrane. The molecular machinery associated with this process may be recruited by specific midgut microvilli membrane domains. To address to this, Spodoptera frugiperda midgut microvillar membranes, prepared by magnesium treatment and free from cytoskeleton with the hyperosmotic Tris procedure, were submitted to detergent extraction and fractionated by density gradient ultracentrifugation. Detergent-resistant membrane domains (DRM) were recovered and their proteins identified by proteomics. Microapocrine vesicles were isolated by washing the luminal surface of the midgut epithelium, followed by freezing and thawing plus centrifugation to recover only membranes. Proteins from purified microvillar membranes and microapocrine vesicle membranes were identified by proteomics. Comparison of the two populations suggests that the budding of microapocrine vesicles surrounded by microvillar membrane is not a random process, because only around 50% of the microvillar membrane proteins are in the microapocrine vesicles. From the 16 proteins from DRM, 14 were enriched in the microapocrine membrane vesicles. These results suggest that on budding, the microapocrine vesicle membrane is enclosed by DRM and a surrounding area of the microvillar membrane. It is proposed that the DRMs somehow recruit the proteins composing the secretory machinery.