THE CONTACT ZONES IN HUMAN ALPHA(2)-MACROGLOBULIN - FUNCTIONAL DOMAINS IMPORTANT FOR THE REGULATION OF THE TRAPPING MECHANISM

A functional domain termed the contact zone, which is the region of a subunit interacting with another non-covalently bound subunit, is sugg ested to play a decisive role in the trapping mechanism of human alpha (2)-macroglobulin. Tetrameric alpha(2)-macroglobulin can be dissociate d into stable dimers with intact thiol esters by sodium thiocyanate, w hereby the contact zones are disrupted. The dissociation leads to sign ificant conformational changes, as studied by ultraviolet-difference s pectroscopy, CD, fluorescence and affinity partitioning. The conformat ion of the dimers is similar to that of MeNH(2)-treated alpha(2)-macro globulin, in which the thiol esters are cleaved, a conformational stat e with a closed trap occurs, and receptor-recognition sites are expose d. The receptor-binding domain is at least partly exposed in the dimer , as judged by binding of specific mAbs. The bait region in the dimers can be cleaved by proteases, and activation of the thiol esters ensue s without binding of the protease. When the dimers were treated with M eNH(2), no conformational changes could be detected by ultraviolet-dif ference spectroscopy or CD. The conformational changes occurring on di ssociation into dimers are suggested to be related to trap closure and receptor-recognition-site exposure without cleavage of the thiol este rs. The model presented here suggests that two separate conformational changes occur in alpha(2)-macroglobulin upon activation. The first in volves changes at the contact zones as a result of the thiol-ester cle avage, and the second causes exposure of the receptor-recognition site s and closure of the trap.