title: Isolation and some properties of macrophage alpha-actinin: evidence that it is not an actin gelling protein.
abstract: We have isolated an actin-binding protein from rabbit alveolar macrophages which by virtue of its physical properties we classify as a nonmuscle alpha-actinin. The protein consists of two subunits of Mr 103 000 and has a Stokes' radius of 7.26 nm and a sedimentation coefficient of 6.83 X 10(-13) s-1. Under the electron microscope, rotary-shadowed molecules appeared as short rods with an average length of 39.9 nm. We have examined the nature of the interaction of macrophage alpha-actinin with F-actin. The binding of radioiodinated macrophage alpha-actinin to F-actin is calcium sensitive. At a low concentration of free calcium (less than 10(-9) M), the binding affinity is 4.2 X 10(6) 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 X 10(6) M-1 with a capacity of four calcium ions per molecule. Although macrophage alpha-actinin has calcium-inhibitable actin gelation activity at 7 degrees C, its effect on the apparent viscosity of F-actin decreases with increasing temperature, and at 37 degrees 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.
mesh_terms: D000185:Actinin / Q000302:isolation & purification* / Q000378:metabolism; D000199:Actins / Q000378:metabolism; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005782:Gels; D046911:Macromolecular Substances; D008264:Macrophages / Q000378:metabolism*; D008854:Microscopy, Electron; D008970:Molecular Weight; D011485:Protein Binding; D011487:Protein Conformation; D011817:Rabbits
chemical_list: D000199:Actins; D000596:Amino Acids; D005782:Gels; D046911:Macromolecular Substances; D000185:Actinin; D002118:Calcium