THE PATTERN OF DISULFIDE LINKAGES IN THE EXTRACELLULAR LOOP REGIONS OF CONNEXIN-32 SUGGESTS A MODEL FOR THE DOCKING INTERFACE OF GAP-JUNCTIONS

Connexins, like true cell adhesion molecules, have extracellular domai ns that provide strong and specific hemophilic, and in some cases, het erophilic interactions between cells. Though the structure of the bind ing domains of adhesion proteins have been determined, the extracellul ar domains of connexins, consisting of two loops of similar to 34-37 a mino acids each, are not easily studied in isolation from the rest of the molecule. As an alternative, we used a novel application of site-d irected mutagenesis in which four of the six conserved cysteines in th e extracellular loops of connexin 32 were moved individually and in al l possible pairwise and some quadruple combinations. This mapping allo wed us to deduce that all disulfides form between the two loops of a s ingle connexin, with the first cysteine in one loop connected to the t hird of the other. Furthermore, the periodicity of movements that prod uced functional channels indicated that these loops are likely to form antiparallel beta sheets. A possible model that could explain how the se domains from apposed connexins interact to form a complete channel is discussed.