Filling in the gap : cloning a connexin

How do you construct a channel between cells? The first step in answering this question was to isolate the components of one type of channel—the gap junction. Daniel Goodenough had been pursuing gap junction proteins since his days as a graduate student in the mid-1960s with Jean-Paul Revel, who helped discover the structures (Revel and Karnovsky, 1967; see “Defining gap junctions” JCB 169:379). In 1972, Goodenough demonstrated that gap junctions could be purified biochemically (Goodenough and Stoeckenius, 1972). By the mid-1970s, when Goodenough had his own lab at Harvard Medical School (Boston, MA), the field was struggling with spotty antibodies in its efforts to identify which proteins were forming the junctions.

When then graduate student David Paul joined Goodenough, they strove to make a clean enough preparation of rat liver gap junctions to generate a better antibody—hopefully one that could be used for expression cloning. Paul says there was nothing “technically miraculous” about their approach, but it was the right time to clone the 1.5 Kb cDNA of a 32-kD gap junction protein (Paul, 1986).

Norton “Bernie” Gilula's lab cloned the same rat liver connexin, now known as connexin 32 (Cx32), the same year (Kumar and Gilula, 1986). This group used an oligonucleotide probe based on a partial protein sequence to pull out cDNAs for the human and then rat liver proteins.

Paul admits that being the first to work with DNA in Goodenough's lab was “scary,” but his molecular leap paid off. Northern blots revealed three related mRNAs—one in liver, brain, kidney, and stomach tissues, and others in heart and lens. That prompted the lab to formally name the family of related proteins as connexins and to clone the genes encoding a heart gap junction protein, Cx43 (Beyer et al., 1987), and a lens gap junction protein, Cx46 (Paul et al., 1991). “Everyone was struggling with the antibodies,” Goodenough recalls, but the cDNAs moved things forward.

The cDNAs also allowed expression experiments in cells that did not normally make gap junctions, thus demonstrating that the proteins could form channels that allowed communication (Dahl et al., 1987). Later studies turned up the sequence-unrelated innexins (invertebrate connexins), the mammalian pannexins (a group more ancient than mammalian connexins), and the insect virus vinnexins.

Connexins were also linked to disease. Cx26 mutations account for about half of all cases of genetic deafness, Cx46 and Cx50 mutations cause familial cataracts, and Cx32 mutations are a common cause of the peripheral nerve disorder Charcot-Marie-Tooth disease (Bergoffen et al., 1993).