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Rapid turnover of nonsense-containing mRNAs in Saccharomyces cerevisiae is dependent on Upf1p, Nmd2p, and Upf3p, the products of the UPF1, NMD2/UPF2, and UPF3 genes, respectively. We showed previously that Upf1p and Nmd2p interact and that this interaction is required for nonsense-mediated mRNA decay (F. He and A. Jacobson, Genes Dev. 9:437-454, 1995; F. He, A. H. Brown, and A. Jacobson, RNA 2:153-170, 1996). In this study we have used the yeast two-hybrid system to define other protein-protein interactions among the essential components of this decay pathway. Nmd2p-Upf3p and Upf1p-Upf3p interactions were identified, and the respective domains involved in these interactions were delineated by deletion analysis. The domains of Upf1p and Upf3p putatively involved in their mutual interaction were found to correspond to the domains on the two proteins which interact with Nmd2p, suggesting that Nmd2p bridges Upf1p and Upf3p. This conclusion was reinforced by experiments showing that: (i) deletion of NMD2 completely abolishes interactions between Upf1p and Upf3p and (ii) overexpression of full-length Nmd2p or Nmd2p fragments that retain Upf1p- and Upf3p-interacting domains promotes 10- to 200-fold enhancement of Upf1p-Nmd2p-Upf3p complex formation. These results; the observation that cells harboring either single or multiple deletions of UPF1, NMD2, and UPF3 inhibit nonsense-mediated mRNA decay to the same extent; and an analysis of the possible targets of a dominant-negative NMD2 allele indicate that Upf1p, Nmd2p, Upf3p, and at least one other factor are functionally dependent, interacting components of the yeast nonsense-mediated mRNA decay pathway.