Ligation of the extracellular domain of the cell surface receptor Fas/APO-1 (CD95) elicits a characteristic programmed death response in susceptible cells. Using a genetic selection based on protein-protein interaction in yeast, we have identified two gene products that associate with the intracellular domain of Fas: Fas itself, and a novel 74 kDa protein we have named RIP, for receptor interacting protein. RIP also interacts weakly with the p55 tumor necrosis factor receptor (TNFR1) intracellular domain, but not with a mutant version of Fas corresponding to the murine lprcg mutation. RIP contains an N-terminal region with homology to protein kinases and a C-terminal region containing a cytoplasmic motif (death domain) present in the Fas and TNFR1 intracellular domains. Transient overexpression of RIP causes transfected cells to undergo the morphological changes characteristic of apoptosis. Taken together, these properties indicate that RIP is a novel form of apoptosis-inducing protein.

PKD1 and PKD2 are two recently identified genes that are responsible for the vast majority of autosomal polycystic kidney disease, a common inherited disease that causes progressive renal failure. PKD1 encodes polycystin, a large glycoprotein that contains several extracellular motifs indicative of a role in cell-cell or cell-matrix interactions, and the PKD2 encodes a protein with homology to a voltageactivated calcium channel and to PKD1. It is currently unknown how mutations of either protein functionally cause autosomal polycystic kidney disease. We show that PKD1 and PKD2 interact through their C-terminal cytoplasmic tails. This interaction resulted in an up-regulation of PKD1 but not PKD2. Furthermore, the cytoplasmic tail of PKD2 but not PKD1 formed homodimers through a coiled-coil domain distinct from the region required for interaction with PKD1. These interactions suggest that PKD1 and PKD2 may function through a common signaling pathway that is necessary for normal tubulogenesis and that PKD1 may require the presence of PKD2 for stable expression.Autosomal dominant polycystic kidney disease (ADPKD) is a common hereditary disease that accounts for 8-10% of endstage renal disease. ADPKD is genetically heterogeneous with loci mapped to chromosome 16p13.3 (PKD1) (1) and to chromosome 4q21-23 (PKD2) (2-4), with the likelihood of a third unmapped locus. PKD1 (5, 6) and PKD2 (4) have recently been cloned and found to be broadly expressed (4, 7). The predicted PKD1 protein is a glycoprotein with multiple transmembrane domains and a C-terminal cytoplasmic tail of 225 amino acids. The N-terminal extracellular region of Ϸ2,557 amino acids contains multiple domains that implicate PKD1 in cell-cell or cell-matrix interactions. These include leucine-rich repeats, a C-type lectin domain, 16 immunoglobulin-like repeats, and 4 type III fibronectin-related domains. PKD2 encodes an integral membrane protein of 968 amino acids containing six transmembrane domains flanked by cytoplasmic N and C termini. Homology of PKD2 to the ␣ 1E-1 subunit of a voltage-activated calcium channel (VACC␣ 1E-1 ) (4) is evident throughout most of the transmembrane domains and the cytoplasmic C-terminal tail, including a potential E-F hand motif. Similarities between PKD1 and PKD2 are restricted to the transmembrane domains I through IV of PKD2. The predicted structures of PKD1 and PKD2, and their similar disease profiles, are highly suggestive of their involvement in a common signaling pathway that links extracellular adhesive events to alterations in ion transport (4).Although various functional abnormalities have been detected in cultured human epithelial cells isolated from cystic lesions of patients with ADPKD, these observations have not clarified the nature of the aberrant gene products caused by mutations of PKD1 and PKD2. Renal cysts are thought to arise through a process of persistent epithelial proliferation related to the lack of terminal differentiation. Both abnormal growth factor responsiveness (8-12) and the elevated e...

The function(s) of the genes (PKD1 and PKD2) responsible for the majority of cases of autosomal dominant polycystic kidney disease is unknown. While PKD1 encodes a large integral membrane protein containing several structural motifs found in known proteins involved in cell-cell or cell-matrix interactions, PKD2 has homology to PKD1 and the major subunit of the voltage-activated Ca 2؉ channels. We now describe sequence homology between PKD2 and various members of the mammalian transient receptor potential channel (TRPC) proteins, thought to be activated by G protein-coupled receptor activation and͞or depletion of internal Ca 2؉ stores. We show that PKD2 can directly associate with TRPC1 but not TRPC3 in transfected cells and in vitro. This association is mediated by two distinct domains in PKD2. One domain involves a minimal region of 73 amino acids in the C-terminal cytoplasmic tail of PKD2 shown previously to constitute an interacting domain with PKD1. However, distinct residues within this region mediate specific interactions with TRPC1 or PKD1. The C-terminal domain is sufficient but not necessary for the PKD2-TRPC1 association. A more N-terminal domain located within transmembrane segments S2 and S5, including a putative pore helical region between S5 and S6, is also responsible for the association. Given the ability of the TRPC to form functional homo-and heteromultimeric complexes, these data provide evidence that PKD2 may be functionally related to TRPC proteins and suggest a possible role of PKD2 in modulating Ca 2؉ entry in response to G protein-coupled receptor activation and͞or store depletion.