UNUSUAL CYTOSKELETAL ASSOCIATION WITH THE 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 fil aments, 7-10 nm in diameter. This web structure receives the microvill ar core rootlets, and is anchored to the lateral plasma membrane at ju nctional level. The filamentous material of the belt structure is not affected by Triton X-100 and cytochalasin B treatments; immunoelectron microscopy with an anti-actin monoclonal antibody did not reveal actin at the web level, while it specifically labelled the microvillar cont ent. The microvillar filamentous core, but not the filamentous materia l of the web, is decorated by S1 fragment of rabbit heavy meromysin. C omparative SDS-gel electrophoresis of Triton-resistant extracts of mid guts from Isotomurus palustris and Orchesella villosa, a species devoi d of the terminal web, showed similar electrophoretic patterns, with a ctin as a major component; however, an additional prominent band at 40 kDa is selectively expressed only in I. palustris, and it has been th erefore considered as the putative component of the terminal web. Whil e the intercellular septa of the smooth septate junction present in mi dgut cells of I. palustris do not seem altered by the presence of the web structure, the intramembrane organization of the junction is modif ied and the typical appearance of the smooth septate junction is not v isible. Many additional intramembranous particles are present on the P -fracture face of the membrane. All the results suggest that the filam entous material of the web may be made up of proteins related to inter mediate filaments; in addition, the adhesion of the web structure to t he plasma membrane substantially alters the appearance of the smooth s eptate junction.