Contractile Protein Isoforms

Abstract

All active cell movement originates in the interaction of only a few different classes of protein molecules. The major representatives are Actin and Tropomyosin, which can assemble into long and thin filamentous polymers, and Myosin, which can also aggregate to form thick filaments. They are found in varying proportions, in the three major categories: namely striated muscle, smooth muscle and non-muscle (or cytoplasmic) contractile system. Biochemical and ultrastructural studies have shown that the proteins from the different contractile systems have many characteristic features in common, and yet measurable differences have been found between them. Contractile proteins are present in numerous Isoforms, which can confer different regulatory and contractile properties to different types of muscle cells. As a consequence the three major categories can be subdivided further in the adult vertebrate systems; and, in addition, some of the components may be present as developmental stage specific (embryonic or neonatal) isoforms. The most frequently used methods, present and past, for the distinction of contractile protein isoforms, include:

• 1. Measurement of mechanical properties of isolated organs, organ strips and “skinned” muscle fibers.

• 2. Myosin ATPase activity in vitro and in situ (histochemistry).

• 3. Electrophoretic analysis of muscle proteins and extracts in the native state, in the denatured state, either alone or in combination with isoelectric focussing.

• 4. Peptide mapping, sequence analysis.

• 5. Immunological assays with antisera ± specific for isoforms.

• 6. Recombinant DNA techniques.