The structure of the pro-apoptotic protease granzyme B reveals the molecular determinants of its specificity

Granzyme B is a serine protease of the chymotrypsin fold that mediates cell death by cytotoxic lymphocytes. It is a processing enzyme, requiring extended peptide substrates containing an Asp residue. The determinants that allow for this substrate specificity are revealed in the three-dimensional structure of granzyme B in complex with a macromolecular inhibitor. The primary specificity for Asp occurs through a side-on interaction with Arg 226, a buried Arg side chain of granzyme B. An additional nine amino acids make contact with the substrate and define the granzyme B extended substrate specificity profile. The substrate determinants found in this structure are shared by other members of this protein class and help to reveal the properties that define substrate specificity.

Characterization of the microheterogeneities of PIXY321, a genetically engineered granulocyte-macrophage colony-stimulating factor/interleukin-3 fusion protein expressed in yeast

PIXY321, a human cytokine analog genetically engineered by the fusion of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3), was expressed in yeast under the control of the alcohol dehydrogenase 2 (ADH2) promoter and the alpha-mating factor expression system. To provide the material necessary for the evaluation of PIXY321 in clinical trials, the production was scaled up to the 1200-1 scale and the PIXY321 molecule isolated by four successive steps of ion-exchange chromatography. Multiple heterogeneities, due to the presence of different patterns of glycosylation as well as multiple amino acid sequences at both N and C termini, were characterized on the purified molecule using complementary analytical techniques including electrophoresis, liquid chromatography and electrospray mass spectrometry. Four different N-terminal sequences were identified but simplified to a reproducible ratio of two sequences, the mature form and a form starting at Ala3, by adjustment of the process conditions. Molecules lacking 1-6 residues at the C-terminus were identified and their relative frequencies quantified. Amino acid modifications, such as three oxidized Met residues at positions 79, 141 and 187 and one deamidated Asn residue at position 176, were detected at low level. Microheterogeneities in glycosylation were characterized on four different sites, one located in the GM-CSF portion and three in the IL-3 portion of the molecule. The sites were shown to be differentially occupied and to carry 0-10 mannose residues according to their location in the sequence. Precise measurement of the heterogeneities at the molecular level were used to tune the process conditions and ensure reproducibility of the clinical product between lots.

Isolation of a proteolytically derived domain of the insulin receptor containing the major site of cross-linking/binding

Radiolabeled insulin was affinity cross-linked to purified insulin receptor with six separate bifunctional N-hydroxysuccinimide esters of different lengths. Results were qualitatively identical for each cross-linker in that insulin was predominantly cross-linked through its B chain to the receptor's alpha subunit. The maximum efficiencies of cross-linking were 10-15% for the most effective reagents, and this value was dependent upon the concentration and length of the cross-linker. In an effort to locate the cross-linking site, monoiodoinsulin was cross-linked to affinity-purified insulin receptor with disuccinimidyl suberate. Limited proteolysis of the hormone/receptor adduct with Staphylococcus aureus V8 protease, chymotrypsin, or thermolysin in an SDS-containing buffer rapidly generated a 55-kDa, insulin-labeled fragment as shown by SDS-polyacrylamide gel electrophoresis. We reported earlier that the 55-kDa chymotryptic fragment contained multiple internal disulfide bonds as evidenced by its shifting mobility on an SDS gel after dithiothreitol treatment [Boni-Schnetzler et al. (1987) J. Biol. Chem. 262, 8395-8401]. Here we show that the 55-kDa fragment is also formed by proteolysis of the receptor in the absence of prior insulin cross-linking. This fragment was prepared in amounts sufficient for sequence analysis and was purified by passage successively over gel permeation and reverse-phase HPLC columns. The sequence of the fragment's amino terminus corresponds to that of the amino terminus of the receptor's alpha subunit. This fragment also reacts with an antibody raised against a synthetic peptide corresponding to residues 242-253 of the receptor's alpha subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin binding changes the interface region between alpha subunits of the insulin receptor

The homobifunctional cross-linking reagent disuccinimidyl suberate (DSS) was used to probe the interface region between the two alpha subunits of the alpha 2 beta 2 human insulin receptor. The two alpha subunits formed a covalent dimer when affinity-purified receptor or membrane-bound receptor was reacted with DSS. The alpha 2 species was detected on protein blots from SDS gels using an anti-alpha-subunit antibody or 125I-concanavalin A. Alternatively, iodinated receptor was reacted with DSS and the alpha 2 species measured directly in an SDS gel. As shown by all three assay systems, more alpha 2 was formed when insulin was bound to receptor than when insulin was absent. These data indicate that the conformational change which occurs in the alpha subunit in response to insulin binding results in a change in the alpha-alpha interaction within the receptor complex. The results are consistent with a kinase activation mechanism involving communication between the two alpha beta receptor halves.

The insulin receptor. Structural basis for high affinity ligand binding

Treatment of the soluble insulin receptor from human placenta with 1.25 mM dithiothreitol and 75 mM Tris at pH 8.5 results in complete reduction of interhalf disulfide bonds (class 1 disulfides) and dissociation of the tetrameric receptor into the dimeric alpha beta form. The alpha beta receptor halves exhibit a reduced affinity for insulin binding (Böni-Schnetzler, M., Rubin, J. B., and Pilch, P. F. (1986) J. Biol. Chem. 261, 15281-15287). Kinetic experiments reveal that reduction of class 1 disulfides is a faster process than the loss of affinity for ligand, indicating that events subsequent to reduction of interhalf disulfides are responsible for the affinity change. We show that a third class of alpha subunit intrachain disulfides is more susceptible to reduction at pH 7.6 than at pH 8.5 and appears to form part of the ligand binding domain. Reduction of the intrachain disulfide bonds in this part of the alpha subunit leads to a loss of insulin binding. Modification of this putative binding domain by dithiothreitol can be minimized if reduction is carried out at pH 8.5. When the insulin receptor in placental membranes is reduced at pH 8.5, the receptor's affinity for insulin is not changed when binding is measured in the membrane. However, the Kd for insulin binding is reduced 10-fold when alpha beta receptor halves are subsequently solubilized. Scatchard analysis of insulin binding to reduced or intact receptors in the membrane and in soluble form together with sucrose density gradient analysis of soluble receptors suggests that alpha beta receptor halves remain associated in the membrane after reduction, but they are dissociated upon solubilization. We interpret these results to mean that the association of two ligand binding domains, 2 alpha beta receptor halves, is required for the formation of an insulin receptor with high affinity for ligand.

Partial characterization of the copolymerization reaction of erythrocyte membrane band 3 with hemichromes

Early intermediates in the denaturation of hemoglobin, termed hemichromes, have been found previously to associate with the cytoplasmic domain of erythrocyte membrane band 3 in a manner which rapidly propagates into an insoluble, macroscopic copolymer. Because this interaction is thought to force a redistribution of band 3 in situ, the properties of the copolymerization reaction were investigated in greater detail. The band 3-hemichrome coaggregate was found to be stabilized largely by ionic interactions since elevation of either ionic strength or pH led to dissolution of the complex. The pH dependence, however, shifted to a more alkaline pH with increasing hemichrome concentration, suggesting a strong linkage between band 3 or hemichrome protonation and copolymer formation. The stoichiometry of the copolymer was measured at five globin chains per band 3 chain whenever underivatized dimer-tetramer hemichrome mixtures were employed. However, cross-linking of the hemichromes at either the alpha or the beta chains to form the stabilized tetramer yielded a copolymer stoichiometry of approximately eight globin chains per band 3 chain, i.e., two hemichrome sites per band 3 subunit. While underivatized hemichromes exhibited both a fast and slow phase of copolymerization, the cross-link-stabilized tetrameric hemichromes displayed predominantly the fast phase kinetics. Naturally occurring disulfide cross-linked hemichromes also reacted more avidly with band 3 than their reduced counterparts; however, the copolymerization process also proceeded to completion with totally reduced components. It is concluded that copolymerization of band 3 with hemichromes should occur under normal cellular conditions and at an accelerated velocity when the intracellular reducing power is low.

Heinz bodies induce clustering of band 3, glycophorin, and ankyrin in sickle cell erythrocytes

In earlier model studies we demonstrated that artificially denatured hemoglobin binds to and clusters the protein, band 3, in the plane of the erythrocyte membrane. To determine whether denatured hemoglobin also clusters band 3 in vivo, we have compared the locations of denatured hemoglobin aggregates (Heinz bodies) with band 3 in sickle cells using phase contrast and immunofluorescence microscopy. We report that where Heinz bodies are found associated with the cytoplasmic surface of the membrane, clusters of band 3 are usually colocalized within the membrane. In contrast, normal erythrocyte membranes and regions of sickle cell membranes devoid of Heinz bodies display an uninterrupted staining of band 3. Similarly, ankyrin and glycophorin are periodically seen to aggregate at Heinz body sites, but the degree of colocalization is lower than for band 3. These data demonstrate that the binding of denatured hemoglobin to the membrane forces a redistribution of several major membrane components.

The role of hemoglobin denaturation and band 3 clustering in red blood cell aging

As hemoglobin begins to denature, it forms hemichromes that cross-link the major erythrocyte membrane-spanning protein, band 3, into clusters. These clusters provide the recognition site for antibodies directed against senescent cells. These antibodies bind to the aged red cell and trigger its removal from circulation.

Hemichrome binding to band 3: nucleation of Heinz bodies on the erythrocyte membrane

Hemichromes, the precursors of red cell Heinz bodies, were prepared by treatment of native hemoglobin with phenylhydrazine, and their interaction with the cytoplasmic surface of the human erythrocyte membrane was studied. Binding of hemichromes to leaky red cell ghosts was found to be biphasic, exhibiting both high-affinity and low-affinity sites. The high-affinity sites were shown to be located on the cytoplasmic domain of band 3, since (i) glyceraldehyde-3-phosphate dehydrogenase, a known ligand of band 3, competes with the hemichromes for their binding sites, (ii) removal of the cytoplasmic domain of band 3 by proteolytic cleavage causes loss of the high-affinity sites, and (iii) the isolated cytoplasmic domain of band 3 interacts tightly with hemichromes, rapidly forming a pH-dependent, water-insoluble copolymer upon mixing in aqueous solution. Since the copolymer of hemichromes with the cytoplasmic domain of band 3 was readily isolatable, a partial characterization of its properties was conducted. The copolymer was shown to be of defined stoichiometry, containing approximately 2.5 hemichrome tetramers (or approximately 5 hemichrome dimers) per band 3 dimer, regardless of the ratio of hemichrome:band 3 in the initial reaction solution. The copolymer was found to be of macroscopic dimensions, generating particles which could be easily visualized without use of a microscope. The coprecipitation was also highly selective for hemichromes, since, in mixed solutions with native hemoglobin, only hemichrome was observed in the isolated pellet. Furthermore, no precipitate was ever observed upon mixing the cytoplasmic domain of band 3 with oxyhemoglobin, deoxyhemoglobin, (carbonmonoxy) hemoglobin, or methemoglobin.(ABSTRACT TRUNCATED AT 250 WORDS)