The tree squirrel HP-25 gene is a pseudogene.
The gene for the hibernation-specific protein HP-25 is expressed in the liver in hibernating species of the squirrel family (chipmunk and ground squirrel), but not in a nonhibernating species (tree squirrel). To investigate why the HP-25 gene is not expressed in the tree squirrel, we isolated the tree squirrel HP-25 gene and compared its gene structure and promoter activity with that of the chipmunk. The tree squirrel HP-25 gene is composed of three exons, and the gene structures are conserved between the tree squirrel and chipmunk. However, the tree squirrel HP-25 gene has an insertional mutation of 13 nucleotides in exon 2 that disrupts the ORF. In the chipmunk HP-25 gene, the 80-bp 5' flanking sequence is sufficient for the liver-specific promoter activity, and HNF-4, which binds to the sequence from nucleotides -67 to -51, is involved in its transcriptional regulation. In contrast, the corresponding tree squirrel 5' flanking sequence had almost no promoter activity in HepG2 cells, and HNF-4 did not bind to the corresponding region of the tree squirrel HP-25 gene. Furthermore, a tree squirrel-type G to A mutation at -57 in the chipmunk HP-25 gene promoter context abolished its binding to and transactivation by HNF-4. Thus, the point mutation in the HNF-4-binding site is likely to be involved in the lack of HP-25 gene expression in the tree squirrel.
Kojima
M
M
Department of Biosciences, School of Science, Kitasato University, Kanagawa Japan.
Shiba
T
T
Kondo
N
N
Takamatsu
N
N
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001798:Blood Proteins / Q000235:genetics* / Q000378:metabolism; D004247:DNA; D004268:DNA-Binding Proteins; D051557:Hepatocyte Nuclear Factor 4; D006605:Hibernation / Q000235:genetics*; D008969:Molecular Sequence Data; D010750:Phosphoproteins / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D011544:Pseudogenes; D012589:Sciuridae; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity; D014157:Transcription Factors / Q000378:metabolism; D015533:Transcriptional Activation
D001798:Blood Proteins; D004268:DNA-Binding Proteins; C081699:HP-25 protein, Tamias asiaticus; D051557:Hepatocyte Nuclear Factor 4; D010750:Phosphoproteins; D014157:Transcription Factors; D004247:DNA