Isolation and sequence determination of cDNA encoding T-protein of the glycine cleavage system

T-protein is a component of the glycine cleavage system and catalyzes the tetrahydrofolate-dependent reaction. A partial cDNA for chicken T-protein was isolated by screening a chicken liver cDNA library with oligonucleotide probes based on amino acid sequences. The clone (CT5C) contains a 675-base pair insert encoding the C-terminal half of the T-protein. Screening of a bovine liver λgt10 cDNA library with the insert of CT5C as a probe detected a clone, BT5A, with an insert of about 2 kilobase pairs. The 1191-base pair coding region encodes a precursor protein of 397 amino acids (M_r 42,852) comprised of a 22-residue mitochondrial targeting sequence and a 375-residue mature protein (M_r 40,534). The 33 amino acids immediately following the targeting sequence correspond exactly to those determined by sequence analysis of the amino terminus of the purified bovine T-protein. The mature protein contains several hydrophilic segments with a cluster of arginine and lysine. The T-protein cDNA probe hybridized to an mRNA species of about 2 kilobases in bovine brain, lung, heart, and liver. A probe for H-protein hybridized with two species of mRNA in these tissues and weak signals were also found in spleen. Although the enzymatic activities of T-protein and H-protein were found in these tissues where transcripts were found, activity of P-protein was detected only in liver and brain. Southern blot analysis of genomic DNA suggested that T-protein is a single copy gene.