CLONING OF A NOVEL NUCLEOLAR GUANOSINE 5'-TRIPHOSPHATE BINDING-PROTEIN AUTOANTIGEN FROM A BREAST-TUMOR

A cDNA clone encoding an immunoreactive autoantigen (Ngp-1) was isolat ed by screening lambda gt11 human ductal breast tumor expression libra ries with autologous patient serum, The complete 2.3-kb nucleotide seq uence of the cDNA was found to contain an open reading frame that coul d encode a protein of 731 amino acids, The predicted amino acid sequen ce contains a high concentration of charged amino acids in the carboxy terminal quarter of the molecule, three guanosine 5'-triphosphate (GT P)-binding protein motifs, and a consensus nuclear localization signal . The arrangement and spacing of the GTP-binding protein motifs indica te that Ngp-1 belongs to a newly described subfamily of GTPases, Excep t for the consensus motifs, neither nucleotide sequence, nor the predi cted amino acid sequence of the Ngp-1 cDNA showed the slightest homolo gy to any vertebrate gene product sequence listed in the databases, No rthern blot analysis showed the 2.3-kb transcript to be ubiquitously e xpressed at relatively low levels in all human tissues tested, with th e highest level of expression in the testes, Immunohistochemical analy sis of tissue sections with affinity-purified antiserum raised against a recombinant Ngp-1 protein revealed that the antigen was exclusively localized to the nucleolus and nucleolar organizer regions in all cel l types analyzed.