Structure and Diversity of Gap Junction Channels

The spatial structure of the channel has been investigated for a long time. In the beginning, light microscopists described intercalated disks which appeared as bands transverse to the longitudinal axis of the cardiac muscle fiber [Eberth, 1866]. With modem phase contrast microscopes they can easily be seen as shown in figure 2. 

These bands were a matter of discussion for a long time until in 1954, for the first time, Sjostrand and Andersson used electron microscopy to investigate intercalated disks in ultrathin osmium tetroxide-fixed sections of the mouse heart revealing that the disks were indeed transverse cell boundaries. Subsequently, several investigators reproduced their finding [Lindner, 1957 Moore 

Using the freeze fracture technique, electron microscopy and laser scanning confocal microscopy, it became obvious that these gap junctional channels are arranged as a cluster of channels with about 50 channels within one disk as stated by Gourdie et al. [1990]. 

From these studies and results Makowsky et al. [1977] developed a three-dimensional model for the channel. According to these studies the gap junction channels are arranged in clusters as shown in figure 3. A model of a single channel is given in figure 4. 

The next question to answer was the mechanism of closure of the channel. It is widely accepted now that the channel is closed by a rotational movement of the hexamer [Unwin and Ennis, 1984 Unwin and Zampighi, 1980] as illustrated in figure 5. This twisting motion closing the central channel is possible since the a-helix of the connexins, which is the part located within the lipid bilayer, is inclined with respect to the axis of the whole connexon [Milks et al., 1988]. 

---

We have considered the structure, diversity and the function of gap junctional channels. But, how are gap junctional channels formed, how are they degraded Or are they not subject to any turnover ... 

---