Glycine cleavage enzyme complex: Rabbit H-protein cDNA sequence analysis and comparison to human, cow, and chicken

The H-protein is one of the four essential components (H-, L-, P-, and T-pr oteins) of the mammalian glycine cleavage enzyme complex, the major degrada tive pathway of glycine. We have isolated the full-length cDNA of the H-pro tein gene from the rabbit (Oryctolagus caniculus) by reverse transcription of liver poly-A mRNA and determined its nucleotide sequence (GenBank Acc. N o. BankIt 318281 AF 231451). Similar to that in human, the rabbit H-protein gene possesses a 519-bp open reading frame that translates a 173-amino-aci d (aa) protein. This reading frame is comprised of a 48-aa mitochondrial ta rgeting sequence and a 125-aa residue that constitutes the mature mitochond rial matrix protein. In the mature protein region, there is a 95.5% nucleot ide and 98.4% amino-acid sequence similarity to human. This conservation wa s also noted in the mature protein of the cow (Bos taurus) and chicken (Gal lus domesticus), where there are a 94.1% and 85.3% nucleotide similarities, and 95.2% and 85.6% amino-acid sequence similarities, respectively. Howeve r, the targeting region is not as well conserved. Comparison of the rabbit targeting sequence to that in human, cow, and chicken reveals 84.0%, 79.2%, and 72.9% nucleotide, and 72.9%, 75.0%, and 54.2% amino-acid sequence simi larities, respectively. These findings suggest that within the H-protein ge ne, the regions encoding the mitochondrial targeting and matrix protein may have evolved differently. Gene diversification in the former may reflect t he species specificity in targeting proteins destined for the mitochondria, whereas homology in the latter suggests a very similar structure-function of the mature H-protein among these species. This homology in structure-fun ction likely accounts for the observation that non-human H-protein can repl ace the human protein in the activity assay of the glycine cleavage enzyme system. This includes the biochemical diagnosis of non-ketotic hyperglycine mia (NKH) resulting from defects other than the H-protein, e.g., mutation(s ) in the P-protein.