THE SHAPE OF HIGH-MOLECULAR-WEIGHT KININOGEN - ORGANIZATION INTO STRUCTURAL DOMAINS, CHANGES WITH ACTIVATION, AND INTERACTIONS WITH PREKALLIKREIN, AS DETERMINED BY ELECTRON-MICROSCOPY

Knowledge of the organization of the kininogen gene and protein struct ure and function correlations has allowed the development of a model o f high molecular weight kininogen. Domains 1-3 on the heavy chain are evolutionarily related to cystatin and the latter two are inhibitors o f cysteine proteases. Proteolytic cleavage in domain 4 to release brad ykinin causes a conformational change, exposing a surface-binding regi on (domain 5) on the disulfide-linked light chain. The carboxyl-termin al domain 6 contains a zymogen binding sequence for factor XI and prek allikrein which, with domain 5, accounts for its cofactor activity. To explore further the domain structure, we have determined the shapes o f high molecular weight kininogen and prekallikrein by electron micros copy of rotary shadowed preparations and computer image processing. Hi gh molecular weight kininogen appears to be a linear array of three li nked globular regions about 16 nm long, with the two ends also connect ed by another thin strand. Both prekallikrein and kallikrein have a co mpact globular shape, with a subdivision that is sometimes visible. Di fferent functional domains of high molecular weight kininogen were ide ntified by monoclonal antibodies against these regions, as well as lig and binding of prekallikrein. These studies indicate that one end glob ular region is the prekallikrein-binding domain, the other comprises t he cysteine protease inhibitor domains and the smaller central nodule is the surface-binding domain. Cleavage of high molecular weight kinin ogen with plasma kallikrein to yield two-chain high molecular weight k ininogen results in a striking change in conformation: the central sur face-binding domain swings out so that it is still adjacent to the pre kallikrein-binding domain but no longer in the middle. These structura l studies provide insight into the interactions of these proteins and aspects of the mechanisms of their actions.