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 isolated by screening λgt11 human ductal breast tumor expression libraries with autologous patient serum. The complete 2.3-kb nucleotide sequence of the cDNA was found to contain an open reading frame that could encode a protein of 731 amino acids. The predicted amino acid sequence contains a high concentration of charged amino acids in the carboxy terminal quarter of the molecule, three guanosine 5′-triphosphate (GTP)-binding protein motifs, and a consensus nuclear localization signal. The arrangement and spacing of the GTP-binding protein motifs indicate that Ngp-1 belongs to a newly described subfamily of GTPases. Except for the consensus motifs, neither nucleotide sequence, nor the predicted amino acid sequence of the Ngp-1 cDNA showed the slightest homology to any vertebrate gene product sequence listed in the databases. Northern blot analysis showed the 2.3-kb transcript to be ubiquitously expressed at relatively low levels in all human tissues tested, with the highest level of expression in the testes. Immunohistochemical analysis 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 cell types analyzed.