Structure Of The Transmembrane Dimer Of Gp55-P Of The Spleen Focus Forming Virus And Its Interaction With The Erythropoietin Receptor

Gp55-P is a dimeric membrane protein with a single transmembrane helix that is coded by the env gene of the polycythemic strain of the spleen focus forming virus. Gp55-P activates the erythropoietin (Epo) receptor through specific transmembrane helix interactions, leading to Epo-independent growth of erythroid progenitors and eventually promoting erythroleukemia. We describe the use of magic angle spinning deuterium NMR to establish the structure of the transmembrane dimer of gp55-P in model membranes. Comparison of the deuterium lineshapes of leucines in the center (Leu396-399) and at the ends (Leu385, Leu407) of the transmembrane sequence shows that gp55-P has a right-handed crossing angle with Leu399 packed in the dimer interface. We extend these NMR studies in two directions. First, deuterium NMR results are presented on Met390 and its interaction with the Epo receptor. Mutation of Met390 to isoleucine (the amino acid at position 390 in gp55-A) eliminates the ability of gp55-P to activate human (Leu238Ser) and mouse Epo receptors. In addition, the M390L mutation was reported to induce anemia, rather than polycythemia. Second, we present 2D dipolar assisted rotational resonance (DARR) NMR measurements of specific helix contacts in the SxxSG sequence that mediate dimerization. We discuss the implications of the structure of the gp55-P transmembrane dimer for activation of the Epo receptor.