AJP - Heart and Circulatory Physiology, Vol 248, Issue 6 783-H791, Copyright © 1985 by American Physiological Society
Cell biology and protein composition of cardiac gap junctions
C. K. Manjunath and E. Page
The gap junctions that electrically couple mammalian myocardial cells have
high (12,000-17,000/micron2)surface densities of channel-containing
elements (connexons), undulating surfaces, and approximately hexagonally
arrayed connexons disposed in small domains rotated with respect to one
another. Optical diffraction combined with image processing of negatively
stained isolated rabbit heart gap junctions shows six protein subunits
surrounding the cell-to-cell channel and suggestive (but not conclusive)
evidence for protein connections between connexons. Biochemical studies
indicate that the six identical relative molecular weight (Mr) 47,000
subunits of mammalian cardiac gap junctions differ from liver gap junctions
in the presence of a covalently bound Mr 17,500 cytoplasmic surface
component that can be visualized electron microscopically in thin-sectioned
and freeze-etched hearts. The cytoplasmic surface component is susceptible
to cleavage by an alkaline serine protease released from mast cell granules
by high ionic strength solutions (0.6 M KI) used to extract myofibrils
during gap junction purification. Interlocking of connexons from coupled
cells in the gap involves hydrogen bonding between protein subunits of the
connexons.
