Lys and Fibrinogen Binding of Wild-Type (Trp72) and Mutant (Arg72) Human Apo(a) Kringle IV-10 Expressed in E coli and CHO Cells

In a previous study, we identified a lysine (Lys)-binding–defective form of human lipoprotein(a) and attributed this defect to the presence of a Trp72→Arg mutation in apolipoprotein(a) [apo(a)] kringle IV-10. To document this relationship, we expressed both wild-type (wt) and mutant (mut) forms of kringle IV-10 inEscherichia coli (nonglycosylated form) and Chinese hamster ovary (CHO) cells (glycosylated form). The Arg72 mut was prepared by introducing the T→A mutation in apo(a) kringle IV-10 amplified from human liver mRNA by the reverse-transcriptase polymerase chain reaction technique. All expressed kringles were tested for their ability to bind Lys and plasmin-modified fibrinogen (PM-fibrinogen). wt kringle IV-10 expressed in bothE coli and CHO cells bound to Lys-Sepharose with comparable affinity. In contrast, the Arg72 mut expressed in both systems exhibited no Lys-binding capacity. Moreover, the wt kringle IV-10 expressed in both systems bound to PM-fibrinogen and exhibited two binding components, one Lys mediated (inhibitable by ε-amino-n -caproic acid) and one Lys insensitive, occurring in about the same proportions. Only the latter type of binding was present in the Arg72 mut expressed inE coli. We conclude that kringle IV-10 of human apo(a) has Lys-and PM-fibrinogen–binding capacities that are independent of glycosylation and require the presence of Trp72, one of the seven amino acids that constitute the Lys-binding site of kringle IV-10. Our results also show that the binding of kringle IV-10 to PM-fibrinogen is more complex than that to Lys, in that the former requires an additional binding site or sites outside the Lys-binding site.