Previously isolated RanBPM, a Ran-binding protein in the microtubule-organizing center, which had been thought to play a role in Ran-stimulated microtubule assembly, turned out to be a truncated protein. To clarify the function of RanBPM, we cloned the full-sized RanBPM cDNA that encodes a 90 kDa protein, compared to the previously isolated cDNA that encoded a 55 kDa protein. The newly cloned 5' coding region contains a great number of cytidine and guanidine nucleotides, like the CpG island. Thus, full-sized RanBPM cDNA encodes a long stretch of proline and glutamine residues in the N-terminal region. It comprises a protein complex of more than 670 kDa. Ran was detected in this complex when RanBPM and Ran were both ectopically expressed. New antibodies to RanBPM were prepared against three different regions of RanBPM. All of them detected a 90 kDa protein that is predominantly localized both in the nucleus and in the cytoplasmic region surrounding the centrosome, but none of them stained the centrosome. In this context, our previous notion that RanBPM is a centrosomal protein should be discarded. RanBPM is well conserved in the animal kingdom. It may play an important role in uncovering Ran-dependent nuclear events.