Intermediate filaments comprise a family of morphologically similar cytoplasmic structures whose individual subunits are biochemically and immunologically distinguishable, and largely cell-type specific. This thesis is an investigation of the intermediate filaments in avian smooth and skeletal muscle, and avian erythrocytes.
Conditions have been found under which sheets of interconnected Z-discs can be generated from skeletal muscle. The stability of these sheets demonstrates that the Z-discs of adjacent myofibrils are firmly linked to one another. Desmin, the intermediate filament subunit characteristic of muscle, encircles each Z-disc and thereby forms an insoluble, two-dimensional scaffold at right angles to the fiber axis within each Z-plane. Desmin may thus be responsible for maintaining the cross-striated appearance of skeletal muscle fibers, and may function to mechanically integrate the contractile actions of their constituent myofibrils. Vimentin, the intermediate filament subunit characteristic of mesenchymal cells, coexists with desmin at the periphery of the Z-disc, and demonstrates that a terminally differentiated cell can possess more than one class of intermediate filament subunit.
A 230,000 dalton polypeptide, named synemin, copurifies with desmin from smooth muscle. It coexists and colocalizes with desmin and vimentin in skeletal muscle at all stages of differentiation, from fusing myoblasts to mature fibers. Nondenaturing conditions under which synemin can be separated from desmin and vimentin have not been found.
Detection of synemin in avian erythrocytes has led to the realization that this cell might be a relatively simple model system for the study of intermediate filaments. A fraction of the intermediate filaments in avian erythrocytes is stably associated with the plasma membrane, but can be selectively removed from it with water; this results in preparations consisting predominantly of vimentin and synemin. Immunoelectron microscopy reveals that vimentin forms the bulk of the core filament in these cells, and synemin exists at regular intervals along this core. The axial periodicity of synemin appears to change during erythropoiesis, perhaps in accordance with some structural or functional change in the filaments. Synemin appears to crosslink the filaments through self-association, and may thus regulate the rigidity or dispersion of the intermediate filament network in erythrocytes as well as in muscle cells.
