cDNA sequence, in vitro synthesis, and intramitochondrial lipoylation of H-protein of the glycine cleavage system.

H-protein is a component of the glycine cleavage system loosely associated with the mitochondrial inner membrane and has lipoic acid as a prosthetic group. cDNA clones encoding H-protein were isolated from a bovine liver cDNA library with an oligonucleotide probe from the amino acid sequence of the NH2-terminal region. DNA sequence analysis and deduced amino acid sequence showed that the cDNAs encoded an H-protein precursor of 173 amino acids including a 48-amino acid presequence. Calculated molecular mass of the precursor and mature protein without lipoic acid were 18,790 and 13,846, respectively. Northern blot analysis indicated a major mRNA component of 1.1 kilobases and a minor component of 0.6 kilobase. The result is consistent with the presence of two adenylation signals in the 3'-untranslated region of the cDNA. In vitro transcription and translation of the H-protein cDNA produced a 19-kDa protein recognized by antibody raised to chicken H-protein. Bovine liver H-protein precursor has no lipoic acid prosthetic group. When incubated with isolated bovine liver mitochondria, the precursor was imported into mitochondria, processed to its mature form with a molecular mass of 14 kDa, and lipoylated at lysine 59. These results indicate that lipoylation is not required for the import of H-protein precursor into mitochondria and H-protein is lipoylated in mitochondria which probably contain the physiologically active form of lipoic acid as well as the enzyme(s) responsible for the attachment.