The mature size of rat 4-aminobutyrate aminotransferase is different in liver and brain

The amino acid sequence predicted from a rat liver cDNA library indicated t hat the precursor of beta-AlaAT I (4-aminobutyrate aminotransferase, beta-a lanine-ortoglutarate aminotransferase) consists of a mature enzyme of 466 a mino acid residues and a. 34-amino acid terminal segment, with amino acids attributed to the leader peptide. However, the mass of beta-AlaAT I from ra t brain was larger than that from rat liver and kidney, as assessed by West ern-blot analysis, mass spectroscopy and N-terminal sequencing. The mature form of beta-AlaAT I from the brain had an ISQAAAK- peptide on the N-termin us of the liver mature beta-AlaAT I. Brain beta-AlaAT I was cleaved to live r beta-AlaAT I when incubated with fresh mitochondrial extract from rat liv er. These results imply that mature rat liver beta-AlaAT I is proteolytical ly cleaved in two steps. The first cleavage of the motif XRX(down arrow)XS is performed by a mitochondrial processing peptidase, yielding an intermedi ate-sized protein which is the mature brain beta-AlaAT I. The second cleava ge, which generates the mature liver beta-AlaAT I, is also carried out by a mitochondrial endopeptidase. The second peptidase is active in liver but l acking in brain.