On the Structural Changes of Native Human α2-Macroglobulin upon Proteinase Entrapment

The reconstructions of an intermediate form of human α2-macroglobulin (half-transformed α2M) in which two of its four bait regions and thiol ester sites were cleaved by chymotrypsin bound to Sepharose were obtained by three-dimensional electron microscopy from stain and frozen-hydrated specimens. The structures show excellent agreement and reveal a structure with approximate dimensions of 195 (length) × 135 (width) and 130 Å (depth) with an internal funnel-shaped cavity. The structure shows that a chisel-shaped body is connected to a broad base at the opposing end by four stands. Four approximately 45 Å diameter large openings in the body of the structure result in a central cavity that is more accessible to the proteinase than those associated with the native or fully transformed structures. The dissimilarity in the shapes between the two ends of α2M half-transformed and the similarity between its chisel-shaped body and that of native α2M indicate that the chymotrypsin has cleaved both bait regions in the bottom-half of the structure. Consequently, its functional division lies on the minor axis. The structural organization is in accord with biochemical studies, which show that the half-transformed α2M migrates on native polyacrylamide gels at a rate intermediate to the native and fully transformed α2M and is capable of trapping 1 mol of proteinase. Even though its upper portion is similar to the native molecule, significant differences in their shapes are apparent and these differences may be related to its slower reaction with a proteinase than the native structure. These structural comparisons further support the view that the transformation of α2M involves an untwisting of its strands with an opening of the cavity for entrance of the proteinase and a retwisting of the strands around the proteinase resulting in its encapsulation.