Isolation and biochemical characterization of septate junctions: Differences between the proteins in smooth and pleated varieties

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Abstract
Septate junctions are found only in invertebrate tissues, and are almost ubiquitous within them. In arthropods, the two major types are the ‘pleated’ and the ‘smooth’ varieties. Using tissues from different species, including the cockroach Periplaneta americana, procedures have been established for obtaining membrane fractions selectively enriched in septate junctions. The junctions have been identified in pellets of these fractions by both thin sectioning and freeze-fracturing. SDS-PAGE of these membrane fractions reveals two major polypeptide species with apparent molecular weights of 22000–24000 and 17000–18000. Consistent differences in these apparent molecular weights are observed between the pleated and smooth varieties of septate junction. These polypeptides are probably integral membrane components, as they remain associated after treatment with high concentrations of urea. Evidence suggests a plane of weakness in the mid-line of the extracellular septal ribbons.