Effect of self-association on the structural organization of partially folded proteins: Inactivated actin

The propensity to associate or aggregate is one of the characteristic prope rties of many nonnative proteins. The aggregation of proteins is responsibl e for a number of human diseases and is a significant problem in biotechnol ogy. Despite this, little is currently known about the effect of self-assoc iation on the structural properties and conformational stability of partial ly folded protein molecules. G-actin is shown to form equilibrium unfolding intermediate in the vicinity of 1.5 M guanidinium chloride (GdmCl). Refold ing from the GdmCl unfolded state is terminated at the stage of formation o f the same intermediate state. An analogous form, known as inactivated acti n, can be obtained by heat treatment, or at moderate urea concentration, or by the release of Ca2+. In all cases actin forms specific associates compr ising partially folded protein molecules. The structural properties and con formational stability of inactivated actin were studied over a wide range o f protein concentrations, and it was established that the process of self-a ssociation is rather specific. We have also shown that inactivated actin, b eing denatured, is characterized by a relatively rigid microenvironment of aromatic residues and exhibits a considerable limitation in the internal mo bility of tryptophans. This means that specific self-association can play a n important structure-forming role for the partially folded protein molecul es.