The structural puzzle of how serpin serine proteinase inhibitors work

alpha1-Antitrypsin Portland, a bioengineered serpin highly selective for furin: application as an antipathogenic agent

The important role of furin in the proteolytic activation of many pathogenic molecules has made this endoprotease a target for the development of potent and selective antiproteolytic agents. Here, we demonstrate the utility of the protein-based inhibitor alpha1-antitrypsin Portland (alpha1-PDX) as an antipathogenic agent that can be used prophylactically to block furin-dependent cell killing by Pseudomonas exotoxin A. Biochemical analysis of the specificity of a bacterially expressed His- and FLAG-tagged alpha1-PDX (alpha1-PDX/hf) revealed the selectivity of the alpha1-PDX/hf reactive site loop for furin (Ki, 600 pM) but not for other proprotein convertase family members or other unrelated endoproteases. Kinetic studies show that alpha1-PDX/hf inhibits furin by a slow tight-binding mechanism characteristic of serpin molecules and functions as a suicide substrate inhibitor. Once bound to furin's active site, alpha1-PDX/hf partitions with equal probability to undergo proteolysis by furin at the C-terminal side of the reactive center -Arg355-Ile-Pro-Arg358- downward arrow or to form a kinetically trapped SDS-stable complex with the enzyme. This partitioning between the complex-forming and proteolytic pathways contributes to the ability of alpha1-PDX/hf to differentially inhibit members of the proprotein convertase family. Finally, we propose a structural model of the alpha1-PDX-reactive site loop that explains the high degree of enzyme selectivity of this serpin and which can be used to generate small molecule furin inhibitors.

Identification and characterization of a serpin with differential expression during the life cycle of the sea lamprey

We have cloned a member of the serine proteinase inhibitor gene superfamily from the sea lamprey, Petromyzon marinus. The predicted translation product contains a putative signal peptide and mRNA expression is localized mainly to the liver. Northern blot analysis indicates that the mRNA increases in the larvae and peaks in late larval life. At the onset of metamorphosis there is a approximately 10-fold drop after which it remains low. These changes correspond with levels of circulating thyroid hormone suggesting that this serpin is involved in or regulated by molecular signals that induce metamorphosis in the lamprey. Use of alignments and structural information from other serpins indicates that the lamprey serpin has the potential to be inhibitory. In addition the lamprey serpin contains methionine and serine at the P1 and P1' positions, respectively. Appropriate residues at positions important in allowing the insertion of strand s4A into beta-sheet A that occurs upon cleavage in inhibitory serpins are also found in the lamprey serpin.

Molecular mechanisms for the conversion of zymogens to active proteolytic enzymes

Proteolytic enzymes are synthesized as inactive precursors, or "zymogens," to prevent unwanted protein degradation, and to enable spatial and temporal regulation of proteolytic activity. Upon sorting or appropriate compartmentalization, zymogen conversion to the active enzyme typically involves limited proteolysis and removal of an "activation segment." The sizes of activation segments range from dipeptide units to independently folding domains comprising more than 100 residues. A common form of the activation segment is an N-terminal extension of the mature enzyme, or "prosegment," that sterically blocks the active site, and thereby prevents binding of substrates. In addition to their inhibitory role, prosegments are frequently important for the folding, stability, and/or intracellular sorting of the zymogen. The mechanisms of conversion to active enzymes are diverse in nature, ranging from enzymatic or nonenzymatic cofactors that trigger activation, to a simple change in pH that results in conversion by an autocatalytic mechanism. Recent X-ray crystallographic studies of zymogens and comparisons with their active counterparts have identified the structural changes that accompany conversion. This review will focus upon the structural basis for inhibition by activation segments, as well as the molecular events that lead to the conversion of zymogens to active enzymes.

Crystal structure of the yeast MATalpha2/MCM1/DNA ternary complex

The structure of a complex containing the homeodomain repressor protein MATalpha2 and the MADS-box transcription factor MCM1 bound to DNA has been determined by X-ray crystallography at 2.25 A resolution. It reveals the protein-protein interactions responsible for cooperative binding of MATalpha2 and MCM1 to DNA. The otherwise flexible amino-terminal extension of the MATalpha2 homeodomain forms a beta-hairpin that grips the MCM1 surface through parallel beta-strand hydrogen bonds and close-packed, predominantly hydrophobic, side chains. DNA bending induced by MCM1 brings the two proteins closer together, facilitating their interaction. An unusual feature of the complex is that an eight-amino-acid sequence adopts an alpha-helical conformation in one of two copies of the MATalpha2 monomer and a beta-strand conformation in the other. This 'chameleon' sequence of MATalpha2 may be important for recognizing natural operator sites.

Three-dimensional structures of proteins involved in programmed cell death

Programmed cell death (apoptosis) is a controlled process by which unwanted cells are selectively eliminated. Several families of proteins including the Bcl-2, tumor necrosis factor receptor 1, and caspase families play essential roles in the regulation, signaling, and execution of the genetic cell death program. The recently described three-dimensional structures of members of these families elucidate the structural basis of their functions and provide insights into the mechanisms by which these proteins regulate apoptosis.

Identification of positively charged residues contributing to the stability of plasminogen activator inhibitor 1

Plasminogen activator inhibitor 1 (PAI-1), a member of the serpins, has a unique conformational flexibility. A typical characteristic is its intrinsic lability resulting in the conversion of the active conformation to a latent conformation. In the present study, we have evaluated the effect of substitution of positively charged residues located at the turn connecting strand s4C with strand s3C, either with negatively charged or with neutral residues, on the functional stability of PAI-1. The following mutants were constructed, purified and characterized in comparison to wild-type (wt) PAI-1: PAI-1-R186E,R187E (Arg186--> Glu and Arg187--> Glu), PAI-1-H190E,K191E (His190--> Glu and Lys191--> Glu) and PAI-1-H190L,K191L (His190--> Leu and Lys191--> Leu). In contrast to wtPAI-1 the mutants exhibited no inhibitory activity. Whereas latent wtPAI-1 can be reactivated (up to a specific activity of 78+/-19%) by treatment with guanidinium chloride, a similar treatment applied to these mutants resulted in a significant but relatively small increase of specific activity (i.e. to 14%). Evaluation of the functional stability (at 37 degrees C, pH 5.5, 1 M NaCl revealed a strongly decreased functional stability compared to wtPAI-1 (i.e. 3-9 h for the mutants vs. > 24 h for wtPAI-1). Further characterization by heat denaturation studies and plasmin susceptibility confirmed that removal or reversal of the positive charge on the turn connecting s4C with s3C results in PAI-1 mutants with a highly accelerated conversion of active to latent forms. We can therefore conclude that the pronounced positive charge in the turn connecting s4C with s3C is of the highest importance for the functional stability of PAI-1.