The distribution of innexin proteins in cells of the pupal salivary gland reflects the highly organised nature of polarised cell structure. The proteins are not distributed randomly about the cell membrane. Instead they occupy a well defined area of the plasma membrane towards the basal end of the cell - at least in larval and pupal salivary glands. The distribution of innexins Ogre and Inx3 also occupy this domain (Inx distribution in pupal salivary gland cells - diagram). The organisation of connexins in polarised vertebrate cells is also restricted to specific areas of the plasma membrane (Guerrier et al. 1995). However, the location of gap junction plaques is tissue dependent, so even though Ogre, Inx2 and Inx3 have a similar distribution in pupal salivary gland cells, they do not in other cell types. For example, in embryonic hindgut epithelial cells Ogre is mainly baso-lateral but Inx2 is more apical (Bauer et al, 2004). A visually similar sub-cellular distribution pattern for different innexin family members does not therefore imply that the different subunits can be found in the same hemichannels (referred to as heteromeric hemichannels). Even in cases where there is abundant evidence that two subunits interact to form heteromeric hemichannels, for example Inx2 + Inx3 (image: Inx3 plaque formation requires Inx2 and Lehmann et al, 2006), plaques are still observed where one of these innexins is present but the other is not (Lehmann et al, 2006). So it appears that only a proportion of the available Inx2 or Inx3 in a cell may be present in the form of heteromeric hemichannels. Not only does the subcellular distribution of innexins differ amongst cell types but even within a given tissue the sub-cellular location and the quantity of gap junction proteins change over the course of the cells life. In embryonic salivary glands innexins are found to be concentrated at the apical end of cells but in the larval and pupal stages the same innexins are mostly found in the basal end of the cell (Image: Diagrammatic comparison of innexin distribution in embryonic and pupal salivary gland cells). A big change in innexin accumulation at plasma membrane plaques can be observed in salivary glands during the change from larval to pupal stage (see webpage - Inx protein levels change over time).
Questions regarding the restricted subcellular distribution of gap junction proteins include:
- - Is there a functional reason for the restricted dispersal of gap junctions in the membrane?
- - Does it aid post-translational regulation (Cruciani and Mikalsen, 2002) of gap junctions or gap junction degradation (Jordan et al. 2001)?
- - other cell-cell junctions are also spatially restricted and this is important for their function. Also, there is evidence of some functional interaction between gap junctions and other cell-cell junctions, such as tight junctions in vertebates (Morita et al. 2004 and Go et al. 2006)....so, is the subcellular location of gap junctions important for the function of other cell-cell junctions?
- - what mechanism targets or restricts gap junctions to the appropriate areas of the plasma membrane?...lipid rafts (Locke et al. 2005)?