Characterization of a gene encoding a Ca(2+)-ATPase-like protein in the plastid envelope.

By screening an Arabidopsis expression library with an antiserum against chloroplast envelope proteins, we have isolated a partial cDNA with an open reading frame that encodes a polypeptide similar to P-type cation-transporting ATPases. The corresponding genomic clone was isolated and the complete coding sequence was deduced after identification and mapping of introns. The gene has been designated PEA1 (plastid envelope ATPase) and the predicted polypeptide PEA1p. PEA1p has 946 amino acids and a molecular mass of 104 kDa. This protein is 40-44% identical to various mammalian plasma membrane Ca(2+)-ATPases but lacks the C-terminal calmodulin binding domain present in the mammalian polypeptides. When aligned with mammalian plasma membrane Ca(2+)-ATPases, PEA1p has a 70- to 80-amino acid N-terminal region that extends beyond the N terminus of these enzymes. This extension has some similarity to the transit peptide of the plastid envelope phosphate translocator and may function to target the protein to the plastid. Antibodies raised against a portion of PEA1p recognize a single 90- to 95-kDa polypeptide in chloroplast inner envelope preparations. Transcript abundance as determined by RNase protection was found to be 7- to 9-fold higher in roots than in leaves. Possible roles for a plastid envelope calcium pump are suggested.

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