The Role of Alpha-Hemoglobin Stabilizing Protein in Human Hemoglobin Assembly

Abstract

Hemoglobin biosynthesis in erythrocyte precursors involves several steps. The correct ratios and concentrations of normal alpha (α) and beta β) globin proteins must be expressed, apoproteins must be folded correctly, heme must be synthesized and incorporated into these globins, and the resulting α and β subunits must be rapidly and correctly assembled into heterotetramers. These events occur on a large scale in vivo, and dysregulation causes serious clinical disorders such as thalassemia syndromes. Recent work has implicated a conserved erythroid protein known as Alpha-Hemoglobin Stabilizing Protein (AHSP) as a participant in these events. Current evidence suggests that AHSP enhances α subunit stability and diminishes its participation in harmful redox chemistry. There is also evidence that AHSP facilitates one or more early-stage post-translational hemoglobin biosynthetic events. In this work, the rate constants associated with AHSP binding to and dissociation from native ferric and ferrous human α subunits have been determined, along with the binding and dissociation equilibrium constants. Also, several mutant AHSP proteins were used to better define the cis-trans peptidyl-prolyl isomerization events that AHSP is known to undergo, and several naturally occurring human a subunit missense mutants were used to probe AHSP function. Additionally, several post-binding events regarding AHSP:α-subunit interactions were investigated, such as autooxidation, heme uptake, hemin loss, effects on ligand binding, and secondary structure acquisition. Finally, AHSP was co-expressed with α and β subunits in transgenic Escherichia coli as a way of probing the effects of AHSP on hemoglobin production. Collectively, these data support the model that AHSP rapidly binds α subunits, stabilizes them, and then is displaced by β subunits during hemoglobin production.