DESCRIPTION (adapted from the application) The clinical importance of Rh (Rhesus) blood group proteins has been recognized for over fifty years. Immune responses to Rh antigens can cause transfusion reactions and hemolytic disease of the newborn. In addition to their importance as targets of immune responses, Rh proteins appear to have an important functional or structural role in the erythrocyte membrane, as individuals who lack expression of all Rh antigens suffer from chronic hemolytic anemia. The eventual, long-term goal of this project is to decipher the function of the Rh proteins, characterize the interactions between the components of the Rh membrane complex, and ultimately, construct an animal model that will explain the clinical effects seen when Rh function is disrupted by either mutation or Rh-specific immune responses. The goal of this proposal is to test the hypothesis, derived from phylogenetic analysis and protein structural predictions, that Rh proteins are involved in ammonium ion transport. This will be approached in Specific Aim 1 by in vivo expression of Rh in Xenopus oocytes and by complementation of a yeast ammonium transport mutant. To address the interaction of the Rh proteins in the erythrocyte membrane in Specific Aim 2, expression and localization of Rh will be characterized in induced and uninduced human and murine cell lines. Cells will be transfected with tagged components of the Rh complex to follow their synthesis and assembly into the complex. These studies will lead, not only to an understanding of Rh function, but also to an explanation of the clinical effects that occur when this function is disrupted.