Physical Characterization of Calponin

Abstract

Calponin is a thin filament-associated smooth muscle protein that has been suggested to play a role in the regulation of smooth muscle contraction. We have used circular dichroism spectroscopy, electron microscopy, and analytical ultracentrifugation to study the physical properties of recombinant chicken gizzard -calponin. The -helix content of -calponin was estimated from its circular dichroism spectrum to be [IMG]/medium/_19960_tex2html_wrap732.gif" ALT="Formula ">13%. -Calponin melts with a single sharp transition at [IMG]/medium/_19960_tex2html_wrap732.gif" ALT="Formula ">57{degrees}C. Rotary shadowing electron micrographs of -calponin reveal diverse shapes ranging from elongated rods to collapsed coils. The lengths of the rod-shaped structures are [IMG]/medium/_19960_tex2html_wrap732.gif" ALT="Formula ">18 nm. Analytical ultracentrifugation studies found -calponin to be homogeneous with a monomer molecular mass of 31.4 kDa, and a s[IMG]/medium/_19960_tex2html_wrap672.gif" ALT="Formula ">[IMG]/medium/_19960_tex2html_wrap566.gif" ALT="Formula ">value of 2.34 S. These data could be used to model -calponin as a prolate ellipsoid of revolution with an axial ratio of 6.16, a length of 16.2 nm, and a diameter of 2.6 nm. Taken together, our results indicate that calponin is a flexible, elongated molecule whose contour length is sufficient to span three actin subunits along the long pitch helix of an F-actin filament.