ABSTRACT
Connexins are the protein subunits o f vertebrate gap junct ions, wh ich are intercellular channels through wh ich ions, metabolites and second messenger molecules below a molecular mass o f about 1000 Da l tons can diffuse. G a p junct ions are organized in most tissues as plaques, wh ich are aggregates consisting o f few to many thousands o f channels. A l t h o u g h these structures look similar when studied by electron microscopy, it was found that connexin proteins are encoded by a large gene family o f at least 15 related genes i n the m a m m a l i a n genome (S imon and Goodenough , 1998; M a n t h e y et al., 1999). These genes are differentially expressed depending on the cell type. Some connexin genes are expressed in very few cell types, others are active in many but not a l l cell types. F o r u n k n o w n reasons, a l l types o f cell appear to express more than one connexin. In ectopic expression systems, l ike X e n o p u s oocytes or transfected m a m mal i an cells (Wil lecke and H a u b r i c h , 1997), connexins can contribute to homotyp ic or heterotypic gap junc t ion channels that are formed by dock ing o f two hemichannels comprised o f the same or different connexin isoforms, respectively. Some types o f connexin hemichannels appear to be incompatible . In addi t ion, at least some o f the connexin proteins can be assembled to heteromeric hemichannels consisting o f dif ferent subunits (Br ink et al., 1997).
