Fusion of isolated myoblast plasma membranes. An approach to the mechanism

Abstract

Summary 105Fusion of plasma membranes isolated from myoblasts grown in culture has been investigated.1. Membrane fusion was specifically dependent on Ca 2 + at physiological concentrations. However, at higher concentrations of cations, fusion could be triggered not only by Ca2+, but by Mg2+ and Sr 2 + as weH.2. The amount of fusion was directly proportional to temperature. 3. Fusion was found to depend on the state of maturation of the my oblast membranes.4. Experiments with chemically and enzymatically modified mem branes and with membranes derived from myoblasts grown in the presence of inhibitors of protein biosynthesis suggest the participation of proteinaceous membrane components in the fusion mechanism.

Surface-labelling studies on skeletal-muscle cells in vitro. Heterogeneity of iodinated cell-surface proteins

“…The loss of this matrix may be reflected in the decrease in iodination of many cell-surface proteins, shown not to co-purify with the plasma-membrane fraction, which was observed in the present study concomitant with and after myoblast fusion. Although the present study indicates that such proteins are not required for the fusion of plasma-membrane vesicles in vitro (Dahl et al, 1978), they may still play a role in facilitating fusion in the intact cultured cell.…”

“…As outlined above, the results obtained in this study do not enable us to define whether those iodinated proteins that do not co-purify with the plasma-membrane fraction, and are altered during myogenesis, represent intrinsic plasma-membrane proteins, peripheral proteins or adsorbed serum components. However, the vesicular plasma-membrane preparation that we have used has been demonstrated to undergo Ca2+dependent fusion in vitro, dependent on the state of maturation of the myoblast membranes used to prepare the plasma-membrane vesicles (Dahl et al, 1978). Regardless of the origin of those iodinated proteins that do not co-purify with the plasma-membrane fraction, and regardless of alterations observed in the iodination of such proteins during myogenesis, we can therefore conclude that these proteins are not required for the fusion process.…”

Experimental depression of junctional membrane permeability in mammalian cell culture. A study with tracer molecules in the 300 to 800 dalton range

“…The cells commonly make junctions, namely gap junctions, at fine processes, cellular protrusions 0.05-0.1 ~tm in diameter and up to 10gin long . These kinds of gap-junctional protrusions have also been seen in other mammalian cell cultures (Dahl, Schudt & Gratzl, 1978;Lawrence et al, 1979). They seem to be specializations of culture.…”

Skeletal Muscle

“…KEETER et al (1975) The gap junction particles are partly exposed. The same factors influence myoblast fusion in vitro ( VAN DER BOSCH et al 1973;DAHL et al 1978;GRATZEL et al 1978). C High magnification of the P face of a gap junction showing that the junctional particles are in an irregular array; 10-nm particles are also seen.…”