We characterize the cDNA and genomic structure of NSBP1, and demonstrate that it is a nuclear protein and the homologue of mouse Nsbp1, which is known to encode a nucleosomal binding and transcriptional activating protein related to the HMG-14/-17 chromosomal proteins. The encoded NSBP1 protein has 86% amino acid similarity to Nsbp1, including identity in nucleosomal binding domains of the HMG-14/-17 proteins. Our radiation hybrid data localize NSBP1 and Nsbp1 to homologous regions of chromosome X, with NSBP1 in Xq13.3 between DXS983 and DXS995 and Nsbp1 in the interval DXMit65 and DXMit39. Although Nsbp1 produces one mRNA transcript, NSBP1 produces three transcripts with alternate polyadenylated sites. The 3′ untranslated region (UTR) of NSPB1 mRNA also contains several AU-rich elements (AREs), which are associated with rapid mRNA turnover. Northern analysis of NSBP1/Nsbp1 shows differences in transcript abundance among adult and fetal tissues, with predominant expression in liver, kidney, trabecular bone, and bone marrow stromal cells. However, a reverse transcriptase-PCR analysis shows nearly ubiquitous expression of the three NSBP1 transcripts in all tissues examined, although the abundance of each transcript was not quantified. NSBP1 is encoded by six exons and has exon-intron boundaries identical to the HMG-14/-17 genes. The last exon and the 3′ UTR of NSBP1 contain retrotransposon sequences of HAL1, HERV-H, and L1MB7, suggesting that these retrotransposons were involved in the origin of NSPB1 from an ancestral-like HMG-14/-17 gene. The similarities among NSBP1, Nsbp1, and the HMG-14/-17 proteins suggest that NSBP1 may function as a nucleosomal binding and transcriptional activating element. Further, the AREs in the 3′ UTR of NSPB1 suggest that alternate poly(A) site selection may mediate the mRNA stability of this gene.
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