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J. Cell Biol.,
Volume 140, Number 5, March 9, 1998 1187-1197
Department of Biological Sciences, State University of New York at Buffalo, Buffalo, New York 14260-1300
Connexins, like true cell adhesion molecules,
have extracellular domains that provide strong and specific homophilic, and in some cases, heterophilic interactions between cells. Though the structure of the binding domains of adhesion proteins have been
determined, the extracellular domains of connexins,
consisting of two loops of ~34-37 amino acids each, are
not easily studied in isolation from the rest of the molecule. As an alternative, we used a novel application of
site-directed mutagenesis in which four of the six conserved cysteines in the extracellular loops of connexin
32 were moved individually and in all possible pairwise and some quadruple combinations. This mapping allowed us to deduce that all disulfides form between the
two loops of a single connexin, with the first cysteine in
one loop connected to the third of the other. Furthermore, the periodicity of movements that produced functional channels indicated that these loops are likely
to form antiparallel 
sheets. A possible model that
could explain how these domains from apposed connexins interact to form a complete channel is discussed.
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