Gap Junctions
Gap junctions are recognized structurally by roughly parallel appositional cell membranes separated by a “gap” of 2–4 nm; in freeze fracture, they appear as plaques of hexameric particles in the membrane (see Scemes and Spray 2008). Three differences between gap junctions structures in vertebrates and invertebrates were noted in early electron microscope studies (Epstein and Gilula 1977; see also Skerrett and Williams 2017). Invertebrate gap junctions have wider “gaps,” larger particles, and tend to cleave with the E face in freeze fracture replicas. The rather tight clustering of the particles at punctate junctional contacts is likely due to the strong and irreversible head to head binding of hexamers (termed connexons, innexons, or pannexons depending on protein composition, see below) that pulls appositional membranes closely together (Fig. 1).
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Scemes, E., Spray, D.C. (2019). Gap Junction Proteins (Connexins, Pannexins, and Innexins). In: Roberts, G., Watts, A. (eds) Encyclopedia of Biophysics. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-35943-9_365-1
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