Alpha-hemoglobin stabilizing protein (AHSP), a kinetic scheme of the action of a human mutant, AHSPV56G.

A kinetic analysis has been made of the interaction of alpha-Hb chains with a mutant alpha-hemoglobin stabilizing protein, AHSP(V56G), which is the first case of an AHSP mutation associated with clinical symptoms of mild thalassemia syndrome. The chaperone AHSP is thought to protect nascent alpha chains until final binding to the partner beta-Hb. Rather than protecting alpha chains, the mutant chaperone is partially unfolded but recovers its secondary structure via interaction with alpha-Hb. For both AHSP(WT) and AHSP(V56G), the binding to alpha-Hb is quite rapid relative to the alpha-beta reaction, as expected because the chaperone binding must be quite competitive to complete the alpha chain folding process before alpha-Hb binds irreversibly to beta-Hb. The main kinetic difference is a dissociation rate of AHSP(V56G).alpha-Hb some four times faster relative to AHSP.alpha-Hb. Considering a role of protein folding, the AHSP(V56G) apparently does not bind long enough (0.5 s versus 2 s for the WT) to complete the structural modifications. The overall replacement reaction (AHSP.alpha-Hb + beta-Hb --> AHSP + alphabeta) can be quite long, especially if there is an excess of AHSP relative to beta-Hb monomers.

Construction of a new polycistronic vector for over-expression and rapid purification of human hemoglobin.

To facilitate the study of the structure-function relationship of human hemoglobin (Hb A), we have developed a new hemoglobin expression vector, pGEX6P-alpha-[SD]-beta. This vector allows the co-expression of alpha-Hb as a fusion protein with Glutathione S-Transferase (GST-alpha-Hb) and beta-Hb with an additional methionine at the N-terminal extremity (rbeta-Hb). These proteins were solubilized as GST-alpha-Hb/rbeta-Hb complex form and purified in one step by affinity chromatography on immobilized glutathione. The CO binding kinetic studies show that the GST-alpha-Hb/rbeta-Hb complex and recombinant Hb A exhibit the same allosteric behavior as for native Hb A. The GST moiety, which does not modify the function of the complex, can be easily eliminated by cleavage by the PreScission Protease. After cleavage during the rapid purification procedure, over 20mg of recombinant Hb per liter of culture were obtained, more than double the yield of previous co-expression systems. This polycistronic vector system, which offers the additional advantage of a very rapid purification, is especially well suited for the study of abnormal, unstable globins in order to better understand the associated pathology.

Interaction of haptoglobin with hemoglobin octamers based on the mutation αAsn78Cys or ßGly83Cys.

Octameric hemoglobins have been developed by the introduction of surface cysteines in either the alpha or beta chain. Originally designed as a blood substitute, we report here the structure and ligand binding function; in addition the interaction with haptoglobin was studied. The recombinant Hbs (rHbs) with mutations alpha Asn78Cys or beta Gly83Cys spontaneously form octamers under conditions where the cysteines are oxidized. Oxygen binding curves and CO kinetic studies indicate a correct allosteric transition of the tetramers within the octamer. Crystallographic studies of the two rHbs show two disulfide bonds per octamer. Reducing agents may provoke dissociation to tetramers, but the octamers are stable when mixed with fresh human plasma, indicating that the reduction by plasma is slower than the oxidation by the dissolved oxygen, consistent with an enhanced stability. The octameric rHbs were also mixed with a solution of haptoglobin (Hp), which binds the dimers of Hb: there was little interaction for incubation times of 15 min; however, on longer timescales a complex was formed. Dynamic light scattering was used to follow the interaction of Hp with the alpha Asn78Cys octamer during 24 hours; a transition from a simple complex of 15 nm to a final size of 60 nm was observed. The results indicate a specific orientation of the αß dimers may be of importance for the binding to haptoglobin.

The vasoactive intestinal peptide-receptor system is involved in human glioblastoma cell migration.

Glioblastoma multiforme (GBM) is the most aggressive form of brain tumor in adults. This cancer has an infiltrative nature and the median survival of patients is about one year. Vasoactive intestinal peptide (VIP) belongs to a structurally related family of polypeptides and is a major regulatory factor in the central and peripheral nervous systems. VIP regulates proliferation of astrocytes and of numerous cancer cell lines and modulates migration in prostatic and colonic cancer cell lines. Little is known about the involvement of VIP and its receptors (VIP-receptor system) in proliferation or migration of GBM cells. The effects of VIP, PACAP and of synthetic VIP antagonists were tested in two human GBM cell lines, M059K and M059J, established from two different parts of a single tumor. In these cells, the data revealed that the VIP-receptor system did not affect proliferation but controlled cell migration. Indeed, in M059K cells which express components of the VIP receptor system, the VIP receptor antagonists and a PACAP antibody enhanced migration. The VIP receptor antagonists increased generation of typical migration-associated processes: filopodia and lamellipodia, and activation of Rac1 and Cdc42 GTPases. Reciprocally, in M059J cells which poorly express the VIP-receptor system, treatments with the agonists VIP and PACAP resulted in decreased cell migration. Furthermore, the peptides appeared to act through a subclass of binding sites displaying an uncommon very high affinity for these ligands. Taken together, these observations suggest that components of the VIP-receptor system negatively regulate cell migration, thus showing potential anti-oncogenic properties.

The alpha-hemoglobin stabilizing protein and expression of unstable alpha-Hb variants.

OBJECTIVES: To determine the role of the alpha-hemoglobin stabilizing protein (AHSP) in the clinical expression of alpha-hemoglobin (alpha-Hb) variants described as unstable, ten alpha chain variants have been studied with their chaperone. AHSP specifically binds free alpha-Hb to form a soluble heterodimer until it is replaced by the beta-Hb partner. In this way, AHSP prevents the precipitation of free alpha chains which might damage the membrane of erythrocyte. AHSP specifically recognizes the G and H helices of alpha-Hb that are also involved in the alpha1beta1 dimer interface. AHSP may act as a modifier in alpha-thalassemias and lead to the thalassemic phenotypes observed in certain unstable alpha-Hb variants previously considered unstable. The different abnormalities of the alpha chain were located either in the G helix: Hb Bronovo alpha103(G10)His-->Leu, Hb Sallanches alpha104(G11)Cys-->Tyr, Hb Oegstgeest alpha104(G11)Cys-->Ser, Hb Bleuland alpha108(G15)Thr-->Asn, Hb Suan Dok alpha109(G16)Leu-->Arg and as yet undescribed alpha109(G16)Leu-->Gln, in the GH corner: Hb Foggia alpha117(GH5)Phe-->Ser, or in the H helix: Hb Groene Hart alpha119(H2)Pro-->Ser, Hb Diamant alpha119(H2)Pro-->Leu, Hb Utrecht alpha129(H12)Leu-->Pro. DESIGN AND METHODS: These different mutated alpha-Hb were co-expressed with their chaperone AHSP as a fusion protein with glutathione S-transferase (GST) and analyzed by SDS-PAGE. RESULTS: In all cases the proteins were normally synthesized in bacteria as shown by an expression level of mutated GST-alpha-Hbs similar to that observed for normal GST-alpha-Hb. In contrast, the recovered quantities of purified mutated GST-alpha-Hbs associated with AHSP are highly variable. An extreme case is GST-alpha-Hb(Utrecht) which was only found at trace levels. CONCLUSION: One can assume that different mechanisms may be responsible for the amount of abnormal Hb recovered, such as a highly unstable alpha chain or an impaired formation of the complex AHSP/alpha-Hb or a modification of the alphabeta dimer formation.