A truncated human RanBPM has been isolated as a protein binding to Ran, Ras-like nuclear small GTPase. Full-sized human RanBPM cDNA which was recently isolated, was found to encode a protein of 90 kDa which comprises a large protein complex. Consistent with this finding, several proteins were found to be co-precipitated with RanBPM by immunoprecipitation analysis. Accordingly, in the present study, we screened the human cDNA library by the two-hybrid method using RanBPM cDNA as bait. One novel protein designated as Twa1 (Two hybrid associated protein No. 1 with RanBPM), and two known proteins, a human homologue (hMuskelin) of mouse Muskelin and HSMpp8 were isolated repeatedly. Twa1 was well conserved through evolution and was localized within the nucleus. Interestingly, in addition to Muskelin and RanBPM, Twa1 was found to possess the LisH-CTLH motif which is detected in proteins involved in microtubule dynamics, cell migration, nucleokinesis and chromosome segregation. These functions overlap with functions suggested for the RanGTPase cycle. Immunoprecipitation and gel-filtration analyses indicated that both Twa1 and hMuskelin did indeed comprise a protein complex with RanBPM. Taken together with the fact that RanBPM interacts with Ran, our present findings suggested that there is an as yet uncovered function of the RanGTPase cycle.