Identification and cDNA cloning of a novel human mosaic protein, LGN, based on interaction with G(alpha i2)

We have used the yeast two-hybrid system to identify proteins that interact with the alpha-subunit of the heterotrimeric GTP-binding protein, G(i2). We screened a human B cell cDNA library with full-length G(alpha i2) and isolated four positive colonies, one of which expressed the 44-kDa COOH terminus of a previously unrecognized 677-amino acid (aa) protein. A full-length clone was isolated from a HeLa cell cDNA library. The deduced protein contains 10 Leu-Gly-Asn repeats, and thus we named it LGN. Computer analysis indicates that LGN is a mosaic protein with seven repeated sequences of about 40 aa in length at its N-terminal end, and four repeated sequences of about 34 aa at its C-terminal end. Each of the two repeat regions shows substantial similarity to proteins found in other organisms. RT-PCR analysis of human tissues showed that the mRNA of LGN was ubiquitously expressed. The specificity of interaction between G(alpha i2) and LGN was confirmed by an in vitro binding assay using recombinant proteins. These data indicate that the yeast two-hybrid system can identify novel proteins, such as LGN, that interact with G(alpha) proteins. As a mosaic protein, LGN shows similarity with portions of proteins from many species and thus may define a new protein family.