Protein 4.1R. (A) Domain map of protein 4.1R and the location of erythrocyte binding partners. (B) Structure of the protein 4.1R membrane binding domain (MBD).⁴⁸  Subdomains where specific proteins bind are colored and labeled. (C) Schematic representation of the protein 4.1R SABD (blue). Two spectrin binding regions straddle an actin binding peptide.⁴⁹  In this hypothetical model, each of the spectrin binding regions is assumed to interact with a different one of the 2 CH domains of β-spectrin, and both CH domains are assumed to interact with actin.⁴⁹  Neither of these assumptions has been proved. Ca²⁺, calcium ion; CaM, calmodulin; PS/PIP₂, phosphatidyl serine/phosphatidylinositol-4,5-bisphosphate. Panel B adapted from Han et al⁴⁸  with permission.