THE ORGANIZATION OF ACTIN IN THE APICAL REGION OF INSECT MIDGUT CELLSAFTER DEEP-ETCHING

The midgut of Tenebrio larvae, which reveals a strong reaction for F-a ctin beneath the apical microvilli after rhodamine-phalloidin treatmen t, was studied to examine localization of actin. Freeze-fracture repli cas of the lateral midgut borders reveal that smooth septate junctions with their characteristic rows of aligned intramembranous particles ( IMPs) are found on the upper third of these borders. Thin sections sho w that short punctate adhering junctions may also occur on this part o f the border. Deep etching reveals that the rows of septate junctional IMPs are closely juxtaposed to cytoplasmic fibrils that demonstrate t he structural features typical of actin as well as heavy meromyosin la beling. These actin fibrils appear to insert into the junctional membr anes. Hence cytoskeletal elements have an intimate spatial association with the membrane modifications typical of intercellular septate junc tions and may be involved in the positioning of their component IMPs a nd also possibly of their septal ribbons. (C) 1998 Academic Press.