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PubMed2002

Are there gap junctions between chief (glomus, type I) cells in the carotid body chemoreceptor? A review.

Since the dye- and electronic couplings between the carotid body chief cells have been demonstrated, the detection and localization of the gap junctions in the carotid body is crucial to understanding the functional mechanism of chemoreception. However, conventional electron microscopy has been unsuccessful in unquestionably detecting ultrastructural features equivalent to the gap junctions, such as close (2 nm in width) membrane appositions in ultrathin sections and aggregations of intramembranous particles in freeze-fracture replicas of the carotid body. We previously reported using a modified electron microscopic study by chemically fixed and subsequent rapid freezing and freeze-substitution method a number of close membrane appositions comparable to the gap junctions. However, we later found that the freeze-substitution also induces numerous close apposition of the membrane in sites where the gap junctions are not known to occur, indicating that the modified electron microscopy by freeze-substitution is not always confirmative in the detection of the gap junction. With regard to the molecular evidence for the gap junction in the carotid body, there have so far been few data on the immunohistochemical demonstration on connexin 32 and 43 in cultured chief cells, but not in the in situ cells.

Animals↗
PubMedDate not supplied

Studies of the structural organization of a bacterial chemoreceptor by electron microscopy.

We used analysis by electron microscopy to obtain structural information about a transmembrane receptor that mediates chemotaxis in Escherichia coli. Two-dimensional arrays of regularly packed particles of the receptor Trg were obtained by reconstitution of purified, detergent-solubilized protein into lipid bilayers. Preliminary image processing of negatively stained arrays revealed an almost square 8.8 x 8.8-nm unit cell and resolved the particles into four peaks of density around a central depression. In certain conditions, reconstituted, Trg-containing bilayers associated into membrane stacks. The regular spacing of the stacks provided a value of 15 nm for the dimension of the receptor normal to the membrane. Using these dimensions, the estimated occupied volume of the structure would be sufficient to contain four monomers of Trg. This tetramer form may be a dimer of two antiparallel or parallel homodimers. Our analysis indicates that a receptor monomer is approximately 4.4 nm at the widest point and 15 nm long. Given the dimensions of the periplasmic domain of the closely related receptor Tars, determined by X-ray crystallography, and a minimum bilayer thickness of 3 nm, the cytoplasmic domain would be approximately 5.0 by 4.4 nm. Higher resolution analysis should reveal additional information about receptor structure.

Bacterial Proteins↗