The interaction between plasminogen and antiplasmin variants as studied by surface plasmon resonance

The Molecular Basis and Biologic Significance of the β-Dystroglycan-Emerin Interaction

β-dystroglycan (β-DG) assembles with lamins A/C and B1 and emerin at the nuclear envelope (NE) to maintain proper nuclear architecture and function. To provide insight into the nuclear function of β-DG, we characterized the interaction between β-DG and emerin at the molecular level. Emerin is a major NE protein that regulates multiple nuclear processes and whose deficiency results in Emery–Dreifuss muscular dystrophy (EDMD). Using truncated variants of β-DG and emerin, via a series of in vitro and in vivo binding experiments and a tailored computational analysis, we determined that the β-DG–emerin interaction is mediated at least in part by their respective transmembrane domains (TM). Using surface plasmon resonance assays we showed that emerin binds to β-DG with high affinity (KD in the nanomolar range). Remarkably, the analysis of cells in which DG was knocked out demonstrated that loss of β-DG resulted in a decreased emerin stability and impairment of emerin-mediated processes. β-DG and emerin are reciprocally required for their optimal targeting within the NE, as shown by immunofluorescence, western blotting and immunoprecipitation assays using emerin variants with mutations in the TM domain and B-lymphocytes of a patient with EDMD. In summary, we demonstrated that β-DG plays a role as an emerin interacting partner modulating its stability and function.

Natural heterogeneity of α2-antiplasmin: functional and clinical consequences

Human α2-antiplasmin (α2AP, also called α2-plasmin inhibitor) is the main physiological inhibitor of the fibrinolytic enzyme plasmin. α2AP inhibits plasmin on the fibrin clot or in the circulation by forming plasmin-antiplasmin complexes. Severely reduced α2AP levels in hereditary α2AP deficiency may lead to bleeding symptoms, whereas increased α2AP levels have been associated with increased thrombotic risk. α2AP is a very heterogeneous protein. In the circulation, α2AP undergoes both amino terminal (N-terminal) and carboxyl terminal (C-terminal) proteolytic modifications that significantly modify its activities. About 70% of α2AP is cleaved at the N terminus by antiplasmin-cleaving enzyme (or soluble fibroblast activation protein), resulting in a 12-amino-acid residue shorter form. The glutamine residue that serves as a substrate for activated factor XIII becomes more efficient after removal of the N terminus, leading to faster crosslinking of α2AP to fibrin and consequently prolonged clot lysis. In approximately 35% of circulating α2AP, the C terminus is absent. This C terminus contains the binding site for plasmin(ogen), the key component necessary for the rapid and efficient inhibitory mechanism of α2AP. Without its C terminus, α2AP can no longer bind to the lysine binding sites of plasmin(ogen) and is only a kinetically slow plasmin inhibitor. Thus, proteolytic modifications of the N and C termini of α2AP constitute major regulatory mechanisms for the inhibitory function of the protein and may therefore have clinical consequences. This review presents recent findings regarding the main aspects of the natural heterogeneity of α2AP with particular focus on the functional and possible clinical implications.

The plasmin-antiplasmin system: structural and functional aspects

The plasmin-antiplasmin system plays a key role in blood coagulation and fibrinolysis. Plasmin and α(2)-antiplasmin are primarily responsible for a controlled and regulated dissolution of the fibrin polymers into soluble fragments. However, besides plasmin(ogen) and α(2)-antiplasmin the system contains a series of specific activators and inhibitors. The main physiological activators of plasminogen are tissue-type plasminogen activator, which is mainly involved in the dissolution of the fibrin polymers by plasmin, and urokinase-type plasminogen activator, which is primarily responsible for the generation of plasmin activity in the intercellular space. Both activators are multidomain serine proteases. Besides the main physiological inhibitor α(2)-antiplasmin, the plasmin-antiplasmin system is also regulated by the general protease inhibitor α(2)-macroglobulin, a member of the protease inhibitor I39 family. The activity of the plasminogen activators is primarily regulated by the plasminogen activator inhibitors 1 and 2, members of the serine protease inhibitor superfamily.

The human alpha(2)-plasmin inhibitor: functional characterization of the unique plasmin(ogen)-binding region

The human alpha(2)-plasmin inhibitor (A2PI) possesses unique N- and C-terminal extensions that significantly influence its biological activities. The C-terminal segment, A2PIC (Asn(398)-Lys(452)), contains six lysines thought to be involved in the binding to lysine-binding sites in the kringle domains of human plasminogen, of which four (Lys(422), Lys(429), Lys(436), Lys(452)) are completely and two (Lys(406), Lys(415)) are partially conserved. Multiple Lys to Ala mutants of A2PIC were expressed in Escherichia coli and used in intrinsic fluorescence titrations with kringle domains K1, K4, K4 + 5, and K1 + 2 + 3 of human plasminogen. We were able to identify the C-terminal Lys(452) as the main binding partner in recombinant A2PIC (rA2PIC) constructs with isolated kringles. We could show a cooperative, zipper-like enhancement of the interaction between C-terminal Lys(452) and internal Lys(436) of rA2PIC and isolated K1 + 2 + 3, whereas the other internal lysine residues contribute only to a minor extent to the binding process. Sulfated Tyr(445) in the unique C-terminal segment revealed no influence on the binding affinity to kringle domains.