ISOLATION AND SOME PROPERTIES OF MACROPHAGE ALPHA-ACTININ - EVIDENCE THAT IT IS NOT AN ACTIN GELLING PROTEIN

An actin-binding protein was isolated from rabbit alveolar macrophages which, by virtue of its physical properties, is classified as a nonmuscle .alpha.-actinin. The protein consists of 2 subunits of MW 103,000 and has a Stokes'' radius of 7.26 nm and a sedimentation coefficient of 6.83 .times. 10-13 s-1. Under the EM, rotary-shadowed molecules appeared as short rods with an average length of 39.9 nm. The nature of the interaction of macrophage .alpha.-actinin with F-actin was examined. The binding of radioiodinated macrophage .alpha.-actinin to F-actin is Ca-sensitive. At a low concentration of free Ca (< 10-9 M), the binding affinity is 4.2 .times. 106 M-1 and is relatively unaffected by changes in temperature, while, in the presence of 0.1 mM Ca2+, binding is reduced more than 5-fold. The stoichiometry of binding suggests that .alpha.-actinin binds all along the length of the actin filaments. The affinity of 45Ca2+ for macrophage .alpha.-actinin is 4 .times. 106 M-1 with a capacity of 4 Ca ions per molecule. Although macrophage .alpha.-actinin has Ca-inhibitable actin gelation activity at 7.degree. C, its effect on the apparent viscosity of F-actin decreases with increasing temperature, and, at 37.degree. C, no gel point is observed. Therefore, at the temperature at which macrophages function in vivo, .alpha.-actinin probably does not promote the isotropic gelation of actin.