KRINGLE-KRINGLE INTERACTIONS IN MULTIMER KRINGLE STRUCTURES

The crystal structure of a monoclinic form of human plasminogen kringl e 4 (PGK4) has been solved by molecular replacement using the orthorho mbic structure as a model and it has been refined by restrained least- squares methods to an R factor of 16.4% at 2.25 Angstrom resolution. T he X-PLOR structure of kringle 2 of tissue plasminogen activator (t-PA K2) has been refined further using PROFFT (R = 14.5% at 2.38 Angstrom resolution). The PGK4 structure has 2 and t-PAK2 has 3 independent mol ecules in the asymmetric unit. There are 5 different noncrystallograph ic symmetry ''dimers'' in PGK4. Three make extensive kringle-kringle i nteractions related by noncrystallographic 2, screw axes without block ing the lysine binding site. Such associations may occur in multikring le structures such as prothrombin, hepatocyte growth factor, plasminog en (PG), and apolipoprotein [a]. The t-PAK2 structure also has noncrys tallographic screw symmetry (3(1)) and mimics fibrin binding mode by h aving lysine of one molecule interacting electrostatically with the ly sine binding site of another kringle. This ligand-like binding interac tion may be important in kringle-kringle interactions involving non-ly sine binding kringles with lysine or pseudo-lysine binding sites. Elec trostatic intermolecular interactions involving the lysine binding sit e are also found in the crystal structures of PGK1 and orthorhombic PG K4. Anions associate with the cationic centers of these and t-PAK2 tha t appear to be more than occasional components of lysine binding site regions.