Our laboratory is interested in defining the structure and properties of the unique class of membrane channels called gap junctions that allow the direct passage of ions, small metabolites and secondary messengers between cells. The proteins that comprise these channels, a family called connexins in the vertebrates , are diverse in nature, with multiple members of the family being expressed in most cells and tissues. It has become increasingly evident that this diversity in connexin composition imparts differential regulatory and permeability properties to these intercellular channels. Understanding the structural basis underlying the different properties of connexins will be an essential step in fully appreciating the specialized role that these structures play in different tissues. Evidence that structural diversity has physiological consequences is provided by the linkage of five very distinct human diseases to defects in different connexin genes. Specifically, deafness is linked to Cx26 and Cx31 mutations, a form of skin keratinopathy is linked to distinct defects in Cx31, peripheral neuronal degeneration in Charcot Marie Tooth's disease is linked to a plethora of Cx32 defects and catarracts are linked to Cx50 defects . Similarly, knock-outs of different connexins in mice have produced highly variable problems ranging from embryonic death (Cx26), to increased susceptibility to tumors (Cx32 ) or cardiac arrhythmias (Cx40), female sterility (Cx37 ) and eye catarracts (Cx46 and 50).

Our own work has contributed significantly to defining patterns of selective interactions between connexins that appear to be important in establishing communication boundaries in vivo. Site-directed mutagenesis, combined with biochemical and functional analyses of connexins expressed in oocyte pairs or cell-free systems, has allowed us to define the structural basis for this "docking" interaction between connexins of apposed cells [Foote etal. J. Cell Biol. 140: 1187 (1998)]. Similar strategies have also been used in probing channel gating mechanisms. Identification of functional domains that are involved in the gating of the channels in response to voltage [Suchyna etal. Nature 365: 847 (1993)] , and phosphorylation by MAPkinase [Zhou etal. J. Cell Biol., 144: 1045 (1999)] has indicated that these processes occur through quite distinct molecular mechanisms. We have also been investigating the different permeability properties of gap junction channels composed of different connexins [Cao etal. J. Cell Science 111: 31 (1998)] . Recently, this has been extended to the identification of natural metabolites that pass preferentially through different channels. In order to identify the determinants for channel selectivity, we have also been employing the SCAM technique of cysteine scanning mutagenesis to identify the domains of the protein that contribute to the channel lining.

The long-term aim of these studies is to better understand the biological role played by gap junctions in different systems. A particular focus is the mechanism by which gap junctions act as tumor suppessors. As part of these studies, we are comparing the permeability of connexins that have proven to be effective growth suppressors , to those that are not. We have also been investigating the mechanism through which some oncogenes (e.g. v-src) can inhibit coupling . This work has shown the mechanism to be like the "ball and chain" gating of K+ channels, in this case instigated by a phosphorylation event, apparently involving MAPkinase. [Zhou etal. J. Cell Biol., 144: 1045 (1999)] This provides us with tools to selectively prevent the uncoupling of cells by v-src, allowing the role of gap junctions in inhibiting the transforming effects of this oncogene to be assessed. 

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To send e-mail: nicholsonb@uthscsa.edu