Unusual cytoskeletal association with intercellular septate junction in the midgut of Collembola (Insecta: Apterygota)

Isotomurus palustris (Apterygota : Collembola) has a midgut epithelium with a discontinuous apical fibrous belt-like structure formed by filaments, 7–10 nm in diameter. This web structure receives the microvillar core rootlets, and is anchored to the lateral plasma membrane at junctional level. The filamentous material of the belt structure is not affected by Triton X-100 and cytochalasin B treatments; immunoelectronmicroscopy with an anti-actin monoclonal antibody did not reveal actin at the web level, while it specifically labelled the microvillar content. The microvillar filamentous core, but not the filamentous material of the web, is decorated by S1 fragment of rabbit heavy meromysin. Comparative SDS-gel electrophoresis of Triton-resistant extracts of midguts from Isotomurus palustris and Orchesella villosa, a species devoid of the terminal web, showed similar electrophoretic patterns, with actin as a major component; however, an additional prominent band at 40 kDa is selectively expressed only in I. palustris, and it has been therefore considered as the putative component of the terminal web. While the intercellular septa of the smooth septate junction present in midgut cells of I. palustris do not seem altered by the presence of the web structure, the intramembrane organization of the junction is modified and the typical appearance of the smooth septate junction is not visible. Many additional intramembranous particles are present on the P-fracture face of the membrane. All the results suggest that the filamentous material of the web may be made up of proteins related to intermediate filaments; in addition, the adhesion of the web structure to the plasma membrane substantially alters the appearance of the smooth septate junction.