CAP binding proteins associated with the nucleus

Cap binding proteins of HeLa cells were identified by photo-affinity labelling using the cap analogue gamma-[32P]-[4-(benzoyl-phenyl)methylamido]-7-methylguanosine-5'- triphosphate. Photoreaction with whole cell homogenates resulted in specific labelling of five major polypeptides. The small molecular weight polypeptide appeared to be identical to the 24 000 to 26 000 dalton cap binding protein previously identified in initiation factors. A cap binding protein of 37 000 dalton was found in initiation factors as well as in preparations of crude nuclei. It was released from nuclei by washing with buffer of moderate salt concentration. Three high molecular weight cap binding proteins (approximately 120 000, approximately 89 000, approximately 80 000 dalton) were found in the nuclear fraction and were only partly released upon nuclease digestion and high salt extraction.