CHARACTERIZATION OF A GENE ENCODING A CA2-ATPASE-LIKE PROTEIN IN THE PLASTID ENVELOPE()

By screening an Arabidopsis expression library with an antiserum again st chloroplast envelope proteins, we have isolated a partial cDNA with an open reading frame that encodes a polypeptide similar to P-type ca tion-transporting ATPases. The corresponding genomic clone was isolate d and the complete coding sequence was deduced after identification an d mapping of introns. The gene has been designated PEA1 (plastid envel ope ATPase) and the predicted polypeptide PEA1p. PEA1p has 946 amino a cids and a molecular mass of 104 kDa. This protein is 40-44% identical to various mammalian plasma membrane Ca2+-ATPases but lacks the C-ter minal calmodulin binding domain present in the mammalian polypeptides. When aligned with mammalian plasma membrane Ca2+-ATPases, PEA1p has a 70- to 80- amino acid N-terminal region that extends beyond the N ter minus of these enzymes. This extension has some similarity to the tran sit peptide of the plastid envelope phosphate trans-locator and may fu nction to target the protein to the plastid. Antibodies raised against a portion of PEA1p recognize a single 90- to 95-kDa polypeptide in ch loroplast inner envelope preparations. Transcript abundance as determi ned by RNase protection was found to be 7- to 9-fold higher in roots t han in leaves. Possible roles for a plastid envelope calcium pump are suggested.