Innexin channel synthesis and transport to the plasma membrane has not been studied in any detail. Connexins, on the other hand, are thought to be synthesized into channels and incorporated into membrane in the golgi apparatus (He et al. 2005) and from there at least two pathways are proposed to exist that transport them to the plasma membrane. One of these pathways is susceptible to treatment with Brefeldin A (BFA) and the other is resistant. Also, the transport pathway taken depends on the connexin subunits comprising the newly synthesised channels. For example, Cx43-containing channels are sensitive to application of BFA but Cx26-containing channels are not (Martin et al. 2001). The small internal innexin-immunopositive vesicles located around the basal end (and it is always the basal, non-lumenal region) of the nucleus in the image above are believed to be newly synthesized channels incorporated into vesicles for transport to the plasma membrane. Qualitatively similar staining is also seen using antibodies against Ogre and Inx3. The innexins Inx2 and Inx3 can form channels together - heteromeric hemichannels (Stebbings et al. 2000) - and one might expect that double-labelling experiments to detect Inx2 and Inx3 should indicate their co-localisation in plasma membrane plaques and in the small transport vesicles.
Putative innexin transport vesicles/sites of gap junction channel synthesis
Figure legend: Image of an early pupal-stage salivary gland cell. This optical section is taken approximately halfway between the cell poles, at the level of the nucleus. Inx2 (green) can be seen in the plasma membrane and around the nucleus in the presumptive golgi complex (arrow). F-actin (red, phalloidin rhodamine), nuclei (blue, DAPI). The internal population of small, nuclear-associated, innexin-immunopositive vesicles is distinct from the larger vesicles (putative annular junctions - see description and close-up images) observed near the basal pole of pupal salivary gland cells.