Conformational change of plasminogen: effects of N-terminal peptides of Glu-plasminogen

Streptococcus co-opts a conformational lock in human plasminogen to facilitate streptokinase cleavage and bacterial virulence

Virulent strains of Streptococcus pyogenes (gram-positive group A Streptococcus pyogenes [GAS]) recruit host single-chain human plasminogen (hPg) to the cell surface-where in the case of Pattern D strains of GAS, hPg binds directly to the cells through a surface receptor, plasminogen-binding group A streptococcal M-protein (PAM). The coinherited Pattern D GAS-secreted streptokinase (SK2b) then accelerates cleavage of hPg at the R561-V562 peptide bond, resulting in the disulfide-linked two-chain protease, human plasmin (hPm). hPm localizes on the bacterial surface, assisting bacterial dissemination via proteolysis of host defense proteins. Studies using isolated domains from PAM and hPg revealed that the A-domain of PAM binds to the hPg kringle-2 module (K2hPg), but how this relates to the function of the full-length proteins is unclear. Herein, we use intact proteins to show that the lysine-binding site of K2hPg is a major determinant of the activation-resistant T-conformation of hPg. The binding of PAM to the lysine-binding site of K2hPg relaxes the conformation of hPg, leading to a greatly enhanced activation rate of hPg by SK2b. Domain swapping between hPg and mouse Pg emphasizes the importance of the Pg latent heavy chain (residues 1-561) in PAM binding and shows that while SK2b binds to both hPg and mouse Pg, the activation properties of streptokinase are strictly attributed to the serine protease domain (residues 562-791) of hPg. Overall, these data show that native hPg is locked in an activation-resistant conformation that is relaxed upon its direct binding to PAM, allowing hPm to form and provide GAS cells with a proteolytic surface.

What the structure of angiostatin may tell us about its mechanism of action

Originally discovered in 1994 by Folkman and coworkers, angiostatin was identified through its antitumor effects in mice and later shown to be a potent inhibitor of angiogenesis. An internal fragment of plasminogen, angiostatin consists of kringle domains that are known to be lysine-binding. The crystal structure of angiostatin was the first multikringle domain-containing structure to be published. This review will focus on what is known about the structure of angiostatin and its implications in function from the current literature.

Lysine-50 is a likely site for anchoring the plasminogen N-terminal peptide to lysine-binding kringles

Interactions between the kringle 4 (K4) domain of human plasminogen (Pgn) and segments of the N-terminal Glu1-Lys77 peptide (NTP) have been investigated via 1H-NMR at 500 MHz. NTP peptide stretches devoid of Lys residues but carrying an internal Arg residue show negligible affinity toward K4 (equilibrium association constant Ka < 0.05 mM(-1)). In contrast, while most fragments containing an internal Lys residue exhibit affinities comparable to that shown by the blocked Lys derivative Nalpha-acetyl-L-lysine-methyl ester (Ka approximately 0.2 mM(-1), peptides encompassing Lys50O consistently show higher Ka values. Among the investigated linear peptides, Nalpha-acetyl-Ala-Phe-Tyr-His-Ser-Ser-Lys5O-Glu-Gln-NH2 (AcAFYHSK5OEQ-NH2) exhibits the strongest interaction with K4 (Ka approximately 1.4 mM(-1)), followed by AcYHSK50EQ-NH2 (Ka approximately 0.9 mM(-1)). Relative to the wild-type sequence, mutated hexapeptides exhibit lesser affinity for K4. When a Lys50 --> Ser mutation was introduced (==> AcYHSS50EQ-NH2), binding was abolished. The Ile27-lle56 construct (L-NTP) contains the Lys50 site within a loop constrained by two cystine bridges. The propensity of recombinant Pgn K1 (rK1) and K2 (rK2) modules, and of Pgn fragments encompassing the intact K4 and K5 domains, for binding L-NTP, was investigated. We find that L-NTP interacts with rK1, rK2, K4, and K5-all lysine-binding kringles-in a fashion that closely mimics what has been observed for the Glul-HSer57 N-terminal fragment of Pgn (CB-NTP). Thus, both the constellation of kringle lysine binding site (LBS) aromatic residues that are perturbed upon complexation of L-NTP and magnitudes of kringle-L-NTP binding affinities (rK1, Ka approximately 4.3 mM(-1); rK2, Ka approximately 3.7 mM(-1; K4, Ka approximately 6.4 mM(1); and K5, Ka approximately 2.1 mM(-1)) are essentially the same as for the corresponding kringle-CB-NTP pairs. Molecular modeling studies suggest that the Glu39-Lys50 stretch in NTP generates an area that complements, both topologically and electrostatically, the solvent-exposed kringle LBS surface.

Evidence that the conformation of unliganded human plasminogen is maintained via an intramolecular interaction between the lysine-binding site of kringle 5 and the N-terminal peptide

Human Glu-plasminogen adopts at least three conformations that provide a means for regulating the specificity of its activation in vivo. It has been proposed previously that the closed (alpha) conformation of human Glu-plasminogen is maintained through physical interaction of the kringle 5 domain and a lysine residue within the N-terminal peptide (NTP). To examine this hypothesis, site-directed mutagenesis was used to generate variant proteins containing substitutions either for aspartic acid residues within the anionic centre of the kringle 5 domain or for conserved lysine residues within the NTP. Size-exclusion HPLC and rates of plasminogen activation by urokinase-type plasminogen activator were used to determine the conformational states of these variants. Variants with substitutions within the kringle 5 lysine-binding site demonstrated extended conformations, as did variants with alanine substitutions for Lys50 and Lys62. In contrast, molecules in which NTP residues Lys20 or Lys33 were replaced were shown to adopt closed conformations. We conclude that the lysine-binding site of kringle 5 is involved in maintaining the closed conformation of human Glu-plasminogen via an interaction with the NTP, probably through Lys50 and/or Lys62. These conclusions advance the current model for the initial stages of fibrinolysis during which fibrin is thought to compete with the NTP for the kringle 5 lysine-binding site.

The kringle domains of human plasminogen

The mature form of the zymogen, human plasminogen (HPlg), contains 791 amino acids present in a single polypeptide chain. The fibrinolytic enzyme, human plasmin (HPlm), is formed from HPlg as a result of activator-catalysed cleavage of the Arg561-Val562 peptide bond in HPlg. The resulting HPlm contains a heavy chain of 561 amino acid residues, originating from the N-terminus of HPlg, doubly disulfide-linked to a light chain of 230 amino acid residues. This latter region, containing the C-terminus of HPlg, is homologous to serine proteases such as trypsin and elastase. The heavy chain of HPlm consists of five repeating triple-disulfide-linked peptide regions, c. 80 amino acid residues in length, termed kringles (K), that are responsible for interactions of HPlg and HPlm with substrates, inhibitors and regulators of HPlg activation. Important among the ligands of the kringles are positive activation effectors, typified by lysine and its analogues, and negative activation effectors, such as Cl-. The kringle domains of HPlg that participate in these binding interactions are K1, K4 and K5, and perhaps K2. These modules appear to function as independent domains. The amino acid residues important in these kringle/ligand binding interactions have been proposed by structural determinations, and their relative importance quantified by site-directed mutagenesis experimentation.

Dual effect of synthetic plasmin substrates on plasminogen activation

The effect of plasmin substrates D-valyl-L-leucyl-lysine-p-nitroanilide (S-2251) and H-D-norleucyl-hexahydrotyrosyl-lysine-p-nitro-anilide (Spectrozyme-PL) on the rate of activation of native human plasminogen in physiological salt solution is studied. Plasminogen activation by two-chain urokinase-type plasminogen activator (urokinase), two-chain tissue-type plasminogen activator (tc-tPA) or trypsin, but not by single chain tPA (sc-tPA) is increased 5- to 10-fold by both substrates, as determined by electrophoretic and spectrophotometric kinetic analysis. The amidolytic activity of sc-tPA, on the other hand, is inhibited by the plasmin substrates in a non-competitive manner (K1 of 6.4 . 10(-4) M for S-2251 and 2.9 . 10(-4) M for Spectrozyme-PL), whereas urokinase and tc-tPA activities are not affected. It is concluded that plasmin substrates containing a lysine residue have a general capacity to enhance plasminogen activation presumably by inducing a conformational change in the native zymogen in a manner similar to 6-aminohexanoate, while the same substrates are inhibitory both on the amidolytic activity of sc-tPA and the activation of native and des1-77-plasminogen by sc-tPA.