Borna disease virus (BDV) causes persistent central nervous system infection and behavioral disturbances in warm-blooded animals. Protein interaction studies were pursued to gain insight into the functions of the putative nucleoprotein (N), phosphoprotein (P), atypical glycoprotein (gp18), and X protein (X) of BDV. Coimmunoprecipitation experiments indicated that N and P, and P and X, form complexes in infected cells. Two-hybrid analyses confirmed interactions between P and P, P and X, and P and N, but not between P and gp18, N and gp18, X and gp18, or X and N. Analysis of P truncation mutants identified three nonoverlapping regions important for oligomerization (amino acids (aa) 135-172), and binding to X (aa 33-115) or N (aa 197-201). Coexpression of X stimulated oligomerization of P but decreased N-P complex formation. Immunocytochemistry of transfected noninfected CHO cells demonstrated that the distribution of X is dependent upon the presence of P- X expressed alone was found predominantly in the cytoplasm whereas coexpression of X and P resulted in nuclear localization. Immunocytochemistry of infected cells revealed nuclear colocalization of P and X. Interactions of P, N, and X may have implications for regulation of BDV transcription/replication and ribonucleoprotein assembly.