"Central fold" or true junctional profile ?

"CENTRAL FOLD" OR TRUE JUNCTIONAL PROFILE? Recently, in an interesting paper entitled "In vitro formation of gap junction vesicles," Daniel Goodenough presented evidence of critical importance to the question of the existence of cell-to-cell channels within the intra-membranous particles of the gap junction. The evidence is based upon Dr. Goodenough's presentation of a folded junctional profile dried on a grid and negatively stained (4; Fig. 5) in which "slender channels approximately 2 nm in diameter appear to span the full 15-rim thickness of the junction profile." The interpretation of these channels as cell-to-cell channels depends upon the identification of this profile as a fold in the edge of a flattened gap junction vesicle as is suggested in the text (reference 4, p. 221-223). We considered, however, the possibility that this folded profile might represent a fold in the center of a flattened gap junction vesicle rather than at an edge, for the following reasons: (a) this fold is not readily identifiable as being at the edge of a gap junction vesicle; (b) it is straight, rather than curved as are most of the edge folds illustrated in Fig. 3 of reference 4; and perhaps more importantly (c) it is discontinuous, each end appearing to taper and merge into the surrounding flattened membrane sheet. We also noted that this profile was approximately twice the width of a true gap junction profile, as would be expected of a central fold in the top layer of a flattened gap junction vesicle. Although the magnifications of Fig. 5 (4) have now been corrected (5) so that the width of this profile and the periodicity of "superposed junctional subunits" approximate these parameters for a true junctional profile, we feel that these measurements would also approximate the expected values for the same parameters of a central fold in a single junctional membrane sheet (or hemi-junction). This possibility is supported by the appearance of apparent central folds of usual junctional width (16-18 nm) in isolated gap junction preparations presented by Benedetti and Emmelot (1) and by Goodenough (3) and by the occurrence of single membrane profiles (hemi-junctions?) in stained thin sections of isolated gap junctions both before (3; Fig. 6) and after (4; Fig. 1) trypsin treatment. If the profile illustrated in Fig. 5 (reference 4 and the accompanying letter) represents a central fold in a single gap junctional sheet, the paired junctional elements implied to be …