The human Y-linked testis determining gene SRY encodes a protein with a DNA binding domain from the high mobility group box family. To date, no function has been assigned to amino acid sequences located outside this DNA binding motif. Here, we identify in a yeast two-hybrid screen a PDZ protein termed SIP-1, as an interacting protein with human SRY. In vitro, biochemical analysis, immunoprecipitation experiments, as well as expression of SIP-1 in human embryonic testis confirm that the two proteins can interact together. Interacting domains were mapped to the C-terminal seven amino acids of SRY and to the PDZ domains of SIP-1, respectively. We hypothesize that SIP-1 could connect SRY to other transcription factors providing SRY for its missing trans-regulation domain.In mammals, male sex determination is controlled by genetic information encoded on the Y chromosome and leads to the differentiation of embryonic gonads into testes. SRY, a Y-specific gene cloned in 1990 (1), was shown to meet all of the criteria of the testis determining factor (2, 3). SRY encodes a small nuclear protein of 204 residues comprising three distinct domains, with a central domain of about 78 amino acids called the high mobility group (HMG) 1 box. This central domain includes a nuclear localization signal (4), and in vitro studies of the human SRY protein have demonstrated its sequence-specific DNA binding through this HMG box (5). To date, no function has been ascribed to the regions of the human SRY protein outside the HMG box, whereas in mouse Sry, the Cterminal part of the protein can function as a transcriptional activator (6). This apparent lack of human SRY transactivation domain and the background-dependent sex determination capabilities of certain mouse Y chromosomes (7) along with the growing list of HMG box containing proteins involved in the assembly of multiprotein complexes (8, 9) suggest that SRY might interact with other proteins. Indeed regulation of target genes by SRY may require collaboration with other factors interacting directly with the SRY protein. Here, using the two-hybrid system we show that a PDZ domain containing nuclear protein interacts with the C-terminal portion of nonrodent SRY. Our two-hybrid results were confirmed by in vitro experiments and by immunoprecipitation that demonstrate that SRY and SIP-1 are associated also in vivo. Finally, immunofluorescence experiments reveal a nuclear expression pattern for SIP-1 in a cell line as well as in human embryo cuts at the level of the genital ridges. These results suggest a model in which SIP-1 could permit to connect SRY to another protein harboring a PDZ binding motif. The nature of this other protein partner and the functional implication of such a complex in human sex determination are now under investigation.
MATERIALS AND METHODSLibrary Screenings-Yeast two-hybrid screening was carried out using the HF7c yeast strain harboring HIS3 and -gal reporter genes under the control of upstream GAL4-binding sites as originally described (10, 11). The bait cons...