Melting of myosin and tropomyosin: electron microscopic observations.

A method was devised to maintain a very low angle (2-3 degrees) during the metal casting of specimens for electron microscopy. With this modified rotary shadowing procedure the melting of myosin and tropomyosin (TM) was investigated. When protein solutions were sprayed on mica sheets and then heated to melt alpha-helices, myosin molecules did not show any sign of chain separation but appeared to have collapsed into loose clumps. A few molecules showed separation of the two chains at the light meromyosin-heavy meromyosin hinge region. Heating myosin in bulk solution at 65 degrees C before spraying caused extensive fusing of the myosin heads. In contrast, in the case of TM, separation of the chains appeared to occur at temperatures at which the unfolding of alpha-helices had been shown by circular dichroism. Dissolution of TM and myosin in 0.5% SDS followed by 150-fold dilution led to single chain species. This method capable of detecting single chain peptides of melting TM whose thickness is of the order of 1 nm may be applicable to the study of the structure of proteins previously not considered possible.