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

Abstract

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.