The t(X;18)(p11.2;q11.2) chromosomal translocation commonly found in synovial sarcomas fuses the SYT gene on chromosome 18 to either of two similar genes, SSX1 or SSX2, on the X chromosome. The SYT protein appears to act as a transcriptional co-activator and the SSX proteins as co-repressors. Here we have investigated the functional domains of the proteins. The SYT protein has a novel conserved 54 amino acid domain at the N-terminus of the protein (the SNH domain) which is found in proteins from a wide variety of species, and a C-terminal domain, rich in glutamine, proline, glycine and tyrosine (the QPGY domain), which contains the transcriptional activator sequences. Deletion of the SNH domain results in a more active transcriptional activator, suggesting that this domain acts as an inhibitor of the activation domain. The C-terminal SSX domain present in SYT-SSX translocation protein contributes a transcriptional repressor domain to the protein. Thus, the fusion protein has transcriptional activating and repressing domains. We demonstrate that the human homologue of the SNF2/Brahama protein BRM co-localizes with SYT and SYT-SSX in nuclear speckles, and also interacts with SYT and SYT-SSX proteins in vitro. This interaction may provide an explanation of how the SYT protein activates gene transcription.

Cystinuria, one of the most common inborn errors of metabolism in humans, accounts for 1-2% of all cases of renal lithiasis. It is caused by defects in the heterodimeric transporter system rBAT/b0,+AT, which lead to reduced reabsorption of cystine and dibasic amino acids through the epithelial cells of the renal tubules and the intestine. In an N-ethyl-N-nitrosourea mutagenesis screen for recessive mutations we identified a mutant mouse with elevated concentrations of lysine, arginine and ornithine in urine, displaying the clinical syndrome of urolithiasis and its complications. Positional cloning of the causative mutation identified a missense mutation in the solute carrier family 3 member 1 gene (Slc3a1) leading to an amino acid exchange D140G in the extracellular domain of the rBAT protein. The mouse model mimics the aetiology and clinical manifestations of human cystinuria type I, and is suitable for the study of its pathophysiology as well as the evaluation of therapeutic and metaphylactic approaches.

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