Cytoplasmic surface ultrastructures of gap junctions in bovine lens fibers.

PURPOSE

To examine the cytoplasmic surface ultrastructures of lens fiber gap junctions, where the cytoplasmic domains of connexons were expected to be exposed.

METHODS

Bovine lens fiber gap junctions, both in situ and in the form of isolated membranes, were examined with the deep etching replica methods. Isolated membranes were also examined with the same methods after the treatment with endoproteinase glu-C, which is known to cleave off the cytoplasmic domain of a putative lens fiber connexin MP70 to determine whether any structural changes should occur between proteolyzed and nonproteolyzed gap junctions. In addition, both proteolyzed and nonproteolyzed gap junctions were studied by sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunolabeling with the monoclonal antibody that recognized cytoplasmic domain of MP70 to clarify whether MP70 lost its cytoplasmic domain by the treatment with endoproteinase glu-C.

RESULTS

Gap junctions were shown to have particulate substructures on their cytoplasmic surfaces; the distributions of the particles were restricted within gap junctional plaques and the non-gap-junctional areas showed smooth cytoplasmic surfaces. Although the treatment with endoproteinase glu-C failed to remove the cytoplasmic particles of gap junctions in deep etching replica study, MP70 was shown to have lost its cytoplasmic domain in sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunolabeling studies.

CONCLUSIONS

Each particle revealed on the cytoplasmic surfaces of lens fiber gap junctions corresponded to the cytoplasmic domain of a connexon. The particles were not removed by the treatment with endoproteinase glu-C, whereas MP70 was cleaved by the same treatment.