Effect of Cytosolic Hereditary Spherocytosis Mutations of Human Erythrocyte Anion Exchanger 1 on Its Interaction with Cytoskeletal Protein 4.2.

Anion exchanger 1 (AE1, Band 3) is the predominant membrane protein of erythrocytes. Human AE1 has two functionally independent domains: its 52 kDa C-terminal membrane domain catalyzes the exchange of chloride for bicarbonate across the membrane while its 43 kDa N-terminal cytosolic domain (cdb3) anchors the membrane to the cytoskeleton, giving the red cell its stability and flexibility. Several proteins bind to cdb3 including cytoskeletal protein 4.2, ankyrin, glycolytic enzymes and deoxyhemoglobin. Three mutations in cdb3 (E40K, G130R and P327R) are associated with the hemolytic anemia hereditary spherocytosis (HS) and decreased levels of erythrocyte protein 4.2 while maintaining a normal amount of AE1 at the red cell membrane. Wild-type and mutant cdb3 proteins were expressed in E. coli and purified and it was shown through a variety of biophysical methods that these three HS mutations do not cause major structural changes in this domain. Each of these mutations introduces a positive charge at the surface of the protein that is predominantly negatively charged, which may have an effect on protein interactions while maintaining its native folded structure. Full-length wild-type AE1 or HS mutants were co-expressed with protein 4.2 in HEK-293 cells in order to study their interaction in a mammalian cell line. All three HS mutant proteins were expressed at similar levels to wild-type in these cells and were shown to be present at the membrane using immunofluorescence and confocal microscopy. These proteins were also co-expressed in LLCPK-1 cells for the purpose of co-localization studies using immunofluorescence. A series of GST fusion proteins of protein 4.2 domains were designed based on a homology model of protein 4.2 and regions of the protein known to interact with AE1. These fusion proteins were expressed in E. coli and purified on glutathione-Sepharose resin and were used to study their interaction with purified wild-type and HS mutant cdb3 proteins in vitro. Blot overlay analysis showed that a protein 4.2 GST fusion protein containing a putative β-hairpin region (Asp145 to Glu203) binds specifically to wild-type cdb3 while GST does not. It is hypothesized that since these three HS mutations do not cause major structural changes in cdb3, the decreased level of protein 4.2 in the red cells of these patients is a result of impaired binding that occurs due to these mutation sites.