PHYSICAL CHARACTERIZATION OF CALPONIN - A CIRCULAR-DICHROISM, ANALYTICAL ULTRACENTRIFUGE, AND ELECTRON-MICROSCOPY STUDY

Calponin is a thin filament-associated smooth muscle protein that has been suggested to play a role in the regulation of smooth muscle contr action. We have used circular dichroism spectroscopy, electron microsc opy, and analytical ultracentrifugation to study the physical properti es of recombinant chicken gizzard alpha-calponin. The alpha-helix cont ent of alpha-calponin was estimated from its circular dichroism spectr um to be similar to 13%. alpha-Calponin melts with a single sharp tran sition at similar to 57 degrees C. Rotary shadowing electron micrograp hs of alpha-calponin reveal diverse shapes ranging from elongated rods to collapsed coils. The lengths of the rod-shaped structures are simi lar to 18 nm. Analytical ultracentrifugation studies found alpha-calpo nin to be homogeneous with a monomer molecular mass of 31.4 kDa, and a s(20,w), value of 2.34 S. These data could be used to model alpha-cal ponin as a prolate ellipsoid of revolution with an axial ratio of 6.16 , a length of 16.2 nn, and a diameter of 2.6 nm. Taken together, our r esults indicate that calponin is a flexible, elongated molecule whose contour length is sufficient to span three actin subunits along the lo ng pitch helix of an F-actin filament.