A 36-kDa protein of the dense bodies of smooth muscle cells.

We purified a 36-kDa protein from a low-salt, alkaline extract of chicken gizzard smooth muscle by sequential column chromatography using DEAE-Cellurofine A-800 m, hydroxylapatite, and CM-Cellurofine C-500. This protein decreased the low-shear viscosity of actin filaments and coprecipitated with them by centrifugation at 18,500 x g. Electron microscopy showed that the 36-kda peptide bundled actin filaments. Immunoblot analysis revealed that an affinity-purified antibody against the 36-kDa protein reacted exclusively with the 36-kDa protein band of smooth muscle. In indirect immunofluorescence microscopy, the affinity-purified anti-36-kDa protein antibody stained the dotty structures of isolated smooth muscle cells, while in post-embedding immunoelectron microscopy, most of the colloidal gold particles representing the 36-kDa protein were found on the dense bodies of ultrathin sections of chicken gizzard smooth muscle cells. The antibody did not stain the dense plaques of isolated smooth muscle cells. Judging from its molecular weight, the 36-kDa protein is concluded to be a new component of the dense bodies of smooth muscle.