Three out of the four residues flanking the glycines in EBC5-16 are bbranched, suggesting that this motif plays a similar role in dimer formation by EBC5-16 and GpA. Prolines are also often present in the middle of transmembrane domains. Because of its rigidity and the absence of a backbone amine hydrogen bond donor, proline can induce a kink in transmembrane sequences, which can allow a conformational change that leads to transmission of a downstream signal. Similarly,Oxysophocarpine Pro22 in the middle of EBC5-16 is essential for activity, and the molecular modeling suggested that it induces a small kink in EBC5-16. The presence of an essential GxxxG packing motif in the homodimer interface and the requirement for a small interfacial amino acid at position 25 for maximal activity provides further support for the hypothesis that tight packing of the EBC5-16 dimer is crucial for its increased activity. In addition to forming a homodimer, EBC5-16 must contain amino acids that mediate activation of the EPOR. The hEPOR is primarily a pre-formed dimer in its inactive state, and a conformational change or rotation of the receptor molecules appears to activate the EPOR in response to EPO binding or genetic manipulations that force the EPOR monomers to adopt a particular orientation. We hypothesize that EBC5-16 induces a similar structural change in the hEPOR, likely through binding directly to the transmembrane domain of the receptor. Strikingly,Diisopropylammonium dichloroacetate addition of the predicted EBC516 interface residues to an inactive poly-leucine construct was sufficient not only for homodimerization but also for activity, demonstrating that these residues restored a functional interaction with the hEPOR. Six leucine residues in the pL-GIPSF are also present in EBC5-16 itself and might interact with the receptor or with another protein that mediates hEPOR activation. Alternatively, one or more of the predicted interface residues may participate in not only homodimer formation but also the interactions required for receptor activation. The surface representation of the CHI model indicates that portions of the interfacial side-chains are accessible at the surface of the dimer for such heteromeric interactions.