Caldesmon is an elongated, flexible molecule localized in the actomyosin domains of smooth muscle.

A rapid purification procedure has been developed for the isolation of caldesmon from hog stomach smooth muscle utilizing a KI extract of washed myofibrils as source material. On SDS-PAGE this mammalian caldesmon showed a closely-spaced doublet around 155 kd. By low-angle rotary shadowing caldesmon was shown to be an elongated, highly flexible molecule which tends to form end-to-end dimers that are structurally very similar to filamin. When added to F-actin solutions caldesmon increased the high-shear viscosity considerably, but by an extent that depended on sample preparation. The effect was shown to be due to caldesmon and not to a trace contaminant by its full reversibility after addition of a monospecific caldesmon antibody. Recent investigations have shown that in smooth muscle two structurally distinct domains can be distinguished: an actomyosin domain and an actin-intermediate filament domain. Immunocytochemistry of ultrathin sections of smooth muscle at the light and electron microscope level revealed that caldesmon is present in the actomyosin domain. Caldesmon is thus a potential regulator of the actomyosin system in smooth muscle.