Molecular analysis of two pyruvate dehydrogenase kinases from maize

Two maize cDNAs were isolated and sequenced that had open reading frames with approximately 37% amino acid identity to mammalian pyruvate dehydrogenase kinases. Both maize kinase sequences contain the five domains with conserved signature residues typical of procaryotic two-component histidine kinases. Sequence comparisons identified six other highly conserved motifs that are proposed to be specific to pyruvate dehydrogenase kinases. In addition, specific Trp and Cys residues are also invariant in these sequences. The maize cDNAs are 1332 (PDK1) and 1602 (PDK2) nucleotides in length, encoding polypeptides with calculated molecular masses of 38,867 and 41,327 Da that share 77% amino acid identity. Reverse transcriptase-polymerase chain reaction analysis with oligonucleotide-specific primers revealed a differential expression pattern for the two isoforms. PDK1 and PDK2 were expressed in Escherichia coli with N-terminal His6 tags to facilitate purification. The recombinant proteins migrated at 44 and 48 kDa, respectively, during SDS-polyacrylamide gel electrophoresis. Anti-PDK1 antibodies immunoprecipitated 75% of pyruvate dehydrogenase kinase activity from a maize mitochondrial matrix fraction, and recognized a matrix protein of 43 kDa. Recombinant PDK2, expressed as a fusion with the maltose-binding protein, inactivated kinase-depleted maize pyruvate dehydrogenase complex when incubated with MgATP, coincident with incorporation of 32P from [gamma-32P]ATP into the alpha subunit of pyruvate dehydrogenase.