Asymmetry of gap junction formation along the animal-vegetal axis of Xenopus oocytes.

Functional expression of gap junction proteins (connexins) in paired oocytes exhibits a strong polar preference: oocyte pairs with their vegetal poles in apposition have higher junctional conductances than equivalent pairs contacting at their animal poles. This asymmetry of cell-cell channel formation is probably due to a corresponding asymmetric distribution of the connexin proteins along the vegetal-animal axis as indicated by immunohistochemical localization. The asymmetry can be influenced by the membrane potential of the oocytes and also by applying an electrical field. A key determinant is the charge of the connexin protein. Mutant connexins in which the net positive charge of the presumed cytoplasmic portion of connexin32 is abolished or reversed to a net negative charge (by addition of negatively charged amino acids to the carboxyl terminus) show loss of asymmetry or reversal of it, respectively. This change of asymmetry of channel formation is paralleled by a change in the distribution of the mutant proteins.