Structure and expression of the mouse growth hormone receptor/growth hormone binding protein gene

Abstract

The mouse growth hormone receptor/growth hormone-binding protein (GHR/BP) gene produces several distinct mRNA forms through alternative splicing, including mRNAs encoding the membrane-bound growth hormone receptor (GHR) and the soluble growth hormone-binding protein (GHBP). Transcripts are also heterogeneous in their 5' regions due to alternative selection of two major 5' untranslated region (5'UTR) sequences, designated L1 and L2. Here we report the cloning of all mouse GHR/BP coding exons as well as the exon encoding 5'UTR L2, the most widely expressed 5'UTR. The mouse GHR/GHBP gene contains 11 coding exons, 9 of which are homologous in size and sequence to human GHR exons 2-10. The two mouse exons that do not have homologs in the human gene are designated exons 4B and 8A. Exon 4B, located between exons 4 and 5, encodes an 8-amino acid segment of the ligand binding domain that is unique to mouse GHR and GHBP. Analysis by reverse transcriptase-polymerase chain reaction indicated that exon 4B is constitutively present in mouse GHR and GHBP mRNA. Exon 8A encodes the GHBP hydrophilic tail and 3'UTR sequence. 5'UTR L2 is encoded by a single exon located at least 27 kb upstream of exon 2 and at least 12 kb upstream of the exon encoding 5'UTR L1. The transcription start sites of UTR L2 were mapped and the 5' flanking region sequenced. The exon and proximal promoter region are GC rich, and share a high level of conservation with the equivalent exons in the sheep, bovine and human GHR genes. A CCAAT motif and several putative Sp1 motifs are present, and there is no TATA box. Homology between the mouse sequence and other species is limited to a region of 450 bp upstream of the exon due to the insertion of a fragment of a LINE-1 element upstream of the mouse L2 exon. Ribonuclease protection assays were used to confirm that 5'UTR L2 is widely expressed in multiple tissues and is the predominant form of transcript except in the liver during pregnancy, in which 5'UTR L1 is the major form.