An abundant erythroid protein that stabilizes free α-haemoglobin

Abstract

The development of red blood cells (erythrocytes) is distinguished by high-level production of the oxygen carrier, haemoglobin A (HbA), a heterotetramer of alpha- and beta-haemoglobin subunits. HbA synthesis is coordinated to minimize the accumulation of free subunits that form cytotoxic precipitates. Molecular chaperones that regulate globin subunit stability, folding or assembly have been proposed to exist but have never been identified. Here we identify a protein stabilizing free alpha-haemoglobin by using a screen for genes induced by the essential erythroid transcription factor GATA-1 (refs 4, 5). Alpha Haemoglobin Stabilizing Protein (AHSP) is an abundant, erythroid-specific protein that forms a stable complex with free alpha-haemoglobin but not with beta-haemoglobin or haemoglobin A (alpha(2)beta(2)). Moreover, AHSP specifically protects free alpha-haemoglobin from precipitation in solution and in live cells. AHSP-gene-ablated mice exhibit reticulocytosis and abnormal erythrocyte morphology with intracellular inclusion bodies that stain positively for denatured haemoglobins. Hence, AHSP is required for normal erythropoiesis, probably acting to block the deleterious effects of free alpha-haemoglobin precipitation. Accordingly, AHSP gene dosage is predicted to modulate pathological states of alpha-haemoglobin excess, such as beta-thalassaemia.

Insights into Hemoglobin Assembly through in Vivo Mutagenesis of α-Hemoglobin Stabilizing Protein

“…AHSP helix 1, helix 2, and the interconnecting loop interact with helices G and H of ␣-globin at a surface that overlaps with the interface for ␤-globin binding. However, ␤ subunits bind ␣ sub-* This work was supported, in whole or in part, by National Institutes of Health units with higher affinity and displace ␣ from AHSP complexes in solution and in vivo (9,10,14,24,25). These observations indicate that AHSP can act as a molecular chaperone to bind nascent ␣-globin and stabilize its folding prior to incorporation into HbA during normal erythropoiesis (12,14,24,25).…”

“…All mice used for experiments were backcrossed onto a C57BL/6 background for five to seven generations. Ahsp Ϫ/Ϫ mice were generated as described previously (10). ␤-Thalassemic (␤-globin Th3/ϩ ) mice were kindly provided by Oliver Smithies (University of North Carolina, Chapel Hill, NC) (48).…”

“…For example, ␣-hemoglobin stabilizing protein (AHSP) binds ␣ but not ␤ chains or intact HbA (9,10) and inhibits denaturation of free ␣-globin in vitro (9,10). Targeted deletion of the Ahsp gene in mice causes globin precipitation, mild hemolytic anemia, and enhanced sensitivity to oxidant stress (10,11).…”

Apoptotic mechanisms in the control of erythropoiesis

“…Erythroid cells possess mechanisms to block a moderate imbalance in globin chain synthesis through the action of the alfa hemoglobin stabilizing protein (AHSP), an abundant erythroid-specific protein that is induced by GATA-1 and forms a stable complex with free a-globin chains. 145 This mechanism of protection of erythroid cells, however, is not sufficient to protect these cells when the globin chain synthesis is greatly unbalanced, as it occurs in thalassemias.…”

A Benign form of HbE/ Beta-Thalassemia

“…Polymorphisms in Xmn1 and BCL11A gene may be associated with increased synthesis of foetal hemoglobin and a milder clinical phenotype [8] . Alpha-hemoglobin stabilising protein gene, a chaperone of α-globin has also been suggested as a modifying factor of the clinical severity, though its role is uncertain [9] . Chronic hyperbilirubinemia, gall bladder disease, and co-inheritance of other hematologic disorders may worsen the clinical phenotype of HbE/β-thalassemia [8] .…”