In BHK 21 cells , the affinity of binding of a protein fragment ( 1 180 ) from __NODE__ protein and a protein fragment ( 175 476 ) containing a ligand binding domain from __SP__ __NODE__ protein is the same as the affinity of binding of a mutant protein fragment ( 1 180 ) ( S89D ) from __NODE__ protein and a protein fragment ( 175 476 ) containing a ligand binding domain from __SP__ __NODE__ protein .