Our studies seek to illustrate a new facet of hemoglobin (Hb) biology. We discovered alpha hemoglobin stabilizing protein (AHSP), an abundant erythroid protein that specifically binds free a globin, maintains its structure and limits its pro-oxidant activities. AHSP and (3 globin compete for binding to the same surface of a globin, although [3 globin binds much more tightly. Therefore, when (3 subunits are present, AHSP is displaced to allow for the formation of HbA (a2|32). Ahsp gene ablation causes hemolytic anemia with Hb precipitates, reflecting an essential role in erythropoiesis. Further preliminary data indicate several distinct molecular functions for AHSP to be explored in separate experimental aims. First, AHSP binds and detoxifies erythroid pools of free ccHb (a globin with heme), as evidenced by biochemical studies, X-ray crystallography of AHSP-aHb complexes and observations that loss of AHSP exacerbates P thalassemia in mice (Aim 1). Second, AHSP appears to serve as a chaperone for newly synthesized apo a globin (a globin without heme), thereby promoting its correct folding and subsequent incorporation into HbA (Aim 2). This function is supported by biochemical studies and our observation that even when the free aHb pool is depleted by a globin gene deletion, AHSP is still required for Hb stability in vivo. In addition, AHSP mRNA contains an iron response element (IRE) that extends transcript half-life when iron is limiting. This suggests potential roles for AHSP in the pathophysiology of iron deficiency or overload (Aim 3). Our overall view is that AHSP utilizes multiple mechanisms to protect against various genetic and environmentally induced imbalances in Hb homeostasis. Investigating these mechanisms will elucidate new basic principles of Hb biology and erythropoiesis. In addition, several practical long-term benefits are possible: First, understanding the functions of AHSP could illustrate novel therapeutic approaches to stabilize deleterious free a globin in human p thalassemias. Second, our studies may provide valuable details as to how AHSP can be exploited as a molecular chaperone to improve the manufacture of recombinant Hb-based artificial blood substitutes.
