MUC1 mucin interacts with calcium-modulating cyclophilin ligand

MUC1 is an integral membrane glycoprotein expressed on epithelial and hematopoietic cells with a COOH-terminus (CT) that mediates intracellular signal transduction. To better understand MUC1-dependent signaling, we searched for proteins binding to its CT using the yeast two-hybrid system with the MUC1 CT as bait and a human epithelial cell cDNA library as prey. Of the six positive clones identified, all encoded calcium-modulating cyclophilin ligand (CAML). The MUC1 CT interacted with CAML in transformed yeast cells as revealed by growth on selective media and in situ X-alpha-galactosidase activity. Binding of the MUC1 CT to CAML in human epithelial cells was confirmed by reciprocal coimmunoprecipitations, confocal microscopy, protein crosslinking, and coupled transcription/translation analyses. By deletion mutagenesis, the NH(2)-terminus of CAML was responsible for binding to the MUC1 CT. Finally, transfection of cells with plasmids encoding MUC1 and CAML increased intracellular calcium levels compared with cells transfected with either plasmid alone, suggesting a possible biological significance of the MUC1-CAML interaction.