Glyoxysomal malate dehydrogenase from watermelon is synthesized with an amino-terminal transit peptide.

The isolation and sequence of a cDNA clone encoding the complete glyoxysomal malate dehydrogenase [gMDH; (S)-malate:NAD+ oxidoreductase, EC 1.1.1.37] of watermelon cotyledons are presented. Partial cDNA clones were synthesized in a three part strategy, taking advantage of the polymerase chain reaction technology with oligonucleotides based on directly determined amino acid sequences. Subsequently, the complete clone for gMDH was synthesized with a sense primer corresponding to the nucleotide sequence of the N-terminal end of pre-gMDH and an antisense primer corresponding to the adenylylation site found in the mRNA. The amino acids for substrate and cofactor binding identified by x-ray crystallography for pig heart cytoplasmic malate dehydrogenase are conserved in the 319-amino-acid-long mature plant enzyme. The pre-gMDH contains an N-terminal transit peptide of 37 residues. It has a net positive charge, lacks a long stretch of hydrophobic residues, and contains besides acidic amino acids a cluster of serine residues. This N-terminal extension is cleaved off upon association with or import into glyoxysomes. It contains a putative AHL topogenic signal for microbody import and has no sequence similarity to the 27-residue-long presequence of the watermelon mitochondrial malate dehydrogenase precursor.