Twenty-Third Annual Meeting February 26–28, 1979 Peachtree Plaza Hotel, Atlanta, Georgia

1979 Biophysical Journal  
Intercellular communication is mediated either by the diffusion of informational molecules through intervening extracellular spaces between cells, or by diffusion of molecules between the cytoplasms of conjoined cells through a regulatable, private pathway which avoids the extracellular space. The plasma membrane specialization which mediates the latter mode of communication is called the gap junction (nexus). This intercellular junction is composed of an assembly of cylindrical subunits
more » ... ons), which span both the joined cells' plasma membranes and a 2-3 nm extracellular gap. Structural studies have set the maximum diameter of the axial channel in each connexon at 2nm. Fluorescent-probe microinjection studies set a 1200 dalton (ow1.4nm) upper limit for anionic molecules which can pass intercellularly through the junctional channels. Under a variety of conditions, most cells have a mechanism to close the junctional channels, switching the gap junctions from a low to a high resistance state, thereby terminating communication with neighboring cells. Coordinated x-ray diffraction and electron microscopy of isolated, high-resistance gap junctions suggest that each connexon spanning the paired lipid bilayers is composed of a dimer of polypeptide hexamers, with a two-fold symmetry plane in the center of the 2nm gap. Trypsin insensitive x-ray diffraction maxima are resolved at 0.47nm on the meridan (electron density profile), which are sampled by an 8nm interference function (separation between bilayer centers). These observations indicate significant 8-conformation in the junctional polypeptides at the level of the membrane hydrophobic interiors. Ultrarapid freezing (2msec) of living tissues on copper surfaces at 4°K reveal dramatic changes in the states of aggregation of the connexons in the membrane plane under different physiological conditions; these differences may be related to changes in junctional resistance. Triads and dyads are junctionsbetween elements of transverse tubular system and sarcoplasmic reticulum in muscle fibers. The junctions are not electrically communicating at resting membrane potentials. Interiors of SR and T-tubules are not in continuity and the ionic composition of the two compartments are different. Following exposure of the muscle to hypertonic solutions the T-tubules swell, thus behaving as extracellular compartments but the SR does not. Depolarization of the T-tubule's membrane above a threshold value results in a large release of calcium to the fibrils. The mechanism of transmission at this junction has not been determined. Morphological evidence indicates that the junction is asymmetric, i.e. that the internal architectures of the two membranes participating in the junction are not identical. The space separating the two membranes is approximately 10 nm wide and is occupied by loosely arranged material which forms bridges (feet) joining the two membranes. The feet are closely associated with the SR membrane and it is likely that they are composed of one or two proteins that are uniquely associated with the heavy microsomal fraction (K. Campbell). Recent results indicate that the feet may be continuous with structures occupying the interior of T-tubules' membranes and thus may provide a direct physical link between the junctional membranes. It is speculated that sites of calcium release are located in the junctional regions of the SR membrane. Supported by MDAA (H.M. Watts Center) and by NIH (HL-15835-08). T-AM-Sy3 INITIAL STEPS IN EXOCYTOSIS AT LIMULUS AMEBOCYTES. R.L. Ornberg and T.S. Reese, Section on Functional Neuroanatomy, LNNS, NINCDS, NIH, Bethesda, Maryland 20014. The initial steps in exocytosis at Limulus amebocytes were captured with rapid-freezing. Amebocytes were used because they degranulate precipitously when exposed to endotoxin, so secretory events can be synchronized; their environment ressembles sea water, so they freeze well; and large numbers are available since they are virtually the only cell in Limulus blood. Furthermore, amebocyte secretory granules are large, so we expected that the processes leading to exocytosis after stimulation with endotoxin might be relatively slow, which would allow us to study them with a rapid-freezing technique that freezes tissues in a few msecs. Prior to endotoxin application, secretory granules at the periphery of amebocytes are separated from the plasmalemma by a narrow region of cytoplasm criss-crossed with fibrous elements seen in both freeze-substituted and freeze-etched cells. At 1-5 secs after endotoxin, the plasmalemma overlying each granule dimples in, closing the gap that separates it from the granule. Then, a small aperture appears at one or, rarely, several points along this apposition and quickly widens to let out the contents of the secretory granule. The presence of fibrous elements where the plasmalemma dimples towards the secretory granule, exactly where exocytosis subsequently begins, suggests that a contractile process is involved in at least the "approach" step of exocytosis in these cells, and that this process may depend in some way on the fibrous connections between the granule and the plasmalemma.
doi:10.1016/s0006-3495(79)85306-0 fatcat:mv3ppk2hd5gk3etv6gf2js46q4