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 closelyspaced 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 diners that are structurally very similar to ifiamin. 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 ifiament 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.

Actin cytoskeletal dynamics in smooth muscle: a new paradigm for the regulation of smooth muscle contraction

“…Small and colleagues (44,52,90) proposed that the smooth muscle actin isoforms ␣ and ␥ preferentially associate with myosin and caldesmon and participate in contraction, whereas the ␤-actin isoform associates with calponin and plays a structural role in smooth muscle cells. Although this idea has been challenged (47), it is of interest to consider whether the polymerization of actin is restricted to a specific actin isoform.…”

Calpain proteolysis of free and bound forms of calponin, a troponin T‐like protein in smooth muscle

“…This may be related to the structural difference between caldesmon and calponin in constituting thin filament. For example, caldesmon is an elongated and highly flexible protein [24][25][26] which would extend over the length of 70-150 nm along the axis of the actin filaments, while calponin may be a globular protein [1,3] which is more deeply buried in actin filaments than caldesmon. Such close association of calponin with actin filament could further support the view that calponin is an integral part of smooth muscle thin filament, which is vital to the actin-linked mechanism of contraction [3,15].…”

Evidence for interaction between smooth muscle tropomyosin and caldesmon

“…The concentration of tropomyosin was determined by the Lowry method using rabbit skeletal tropomyosin as a standard [23] and that of caldesmon was determined from the absorbance at 276 nm [14]. The reported molecular mass of a caldesmon polypeptide chain has ranged from 120 to 150 kDa [5,8,14,24,25]. In this study the molecular mass of a caldesmon chain was taken to be 140 kDa and that of the tropomyosin molecule to be 66 kDa.…”

“…These results indicate that tropomyosin and caldesmon bind under these low-ionic conditions, resulting in an increased viscosity, and furthermore suggest a binding stoichiometry of two caldesmon polypeptide chains per tropomyosin molecule. At present it is controversial whether the caldesmon molecule is a monomer or dimer [5,14,25,261. The ratio of caldesmon polypeptide chains to tropomyosin when both proteins are bound to actin is, however, less than 2.0.…”