DNA sequencing of a mouse liver protein that binds selenium: implications for selenium's mechanism of action in cancer prevention.

Complementary DNA clones for liver protein 56K (SLP-56) were isolated by screening a lambda Zap mouse liver library. The cloned cDNAs represented the complete message. The correct reading frame was verified by alignment of the deduced amino acid sequence with that of peptides sequenced from the purified protein. The primary sequence has not been reported previously since homologous DNA sequences were not found in GenBank. Most importantly, the DNA sequence did not contain an in-frame TGA codon that would code for seleno-cysteine, as occurs in the prototypic selenoprotein, glutathione peroxidase. Hydropathy analysis suggested the protein was not a membrane-spanning protein. SLP-56 was previously localized as a cytosolic-soluble protein on the basis of cell fractionation experiments. The results suggest that SLP-56 is different from proteins whose synthesis and concentration are dependent upon selenium and require TGA to encode for selenocysteine. In this respect, SLP-56 appears to be similar to liver fatty acid binding protein (SLP-14) for which selenium is a ligand. Our working hypothesis is that selenium exerts its inhibitory effects on cell growth by modulating the properties of existing growth regulatory proteins. The two proteins that are readily labeled by selenium in many rodent tissues, SLP-56 and SLP-14, would fit into this category.