Protein diffusivities in skinned frog skeletal muscle fibers.

In this preliminary report, we investigate the problem of why proteins appear to diffuse much more slowly in muscle cytoplasm than in aqueous solutions. Microvolumetric sampling methods were used to partition the cytoplasm of freshly skinned muscle fibers into diffusible (cytosol) and nondiffusible (cytomatrix) fractions. Some of the diffusible proteins that were identified tentatively on the basis of electrophoretic mobility are: glyceraldehyde-3-phosphate dehydrogenase, triose-P-isomerase, phosphoglycerate mutase and parvalbumin. The latter was most abundant. The apparent (radial) diffusion coefficients of these proteins in the skinned fiber were about a tenth (or less) of their value in aqueous solution. Tortuosity and viscosity factors are probably responsible for their reduced diffusivity, but binding to cytomatrix proteins or sarcoplasmic reticular structures may also contribute. The apparent diffusion coefficient of parvalbumin, a strong chelator of Ca2+, was similar to that reported for trace amounts of Ca2+ injected into skinned frog fibers under oil1), thus supporting the notion that the low apparent diffusion coefficient of Ca2+ is actually that of a Ca2+-parvalbumin complex.