A protein kinase family in Arabidopsis phosphorylates chloroplast precursor proteins.
A serine/threonine protein kinase that is able to phosphorylate chloroplast-destined precursor proteins was purified from leaf extract of Arabidopsis thaliana and was identified by mass spectrometry. The protein kinase, encoded by AT2G17700, belongs to a small protein family comprising in addition AT4G35780 and AT4G38470. All three proteins were expressed heterologously in Escherichia coli and characterized with regard to their properties in precursor protein phosphorylation. They were able to phosphorylate several chloroplast-destined precursor proteins within their cleavable presequences. In contrast, a mitochondria-destined precursor protein was not a substrate for these kinases. For all three enzymes, the phosphorylation reaction was specific for ATP with apparent K(m) values between 14 and 67 microM. They did not utilize other NTPs nor were those able to compete for ATP in the reaction. An excess of ADP was able to inhibit ATP-dependent phosphorylation. Furthermore, all three kinases exhibited autophosphorylation. The protein kinases described here could represent subunits of a regulatory network involved in the cytosolic events of chloroplast protein import.
Martin
Torsten
T
Department Biology I, Botany, Ludwig-Maximilians-University Munich, Menzingerstrasse 67, D-80638 Munich, Germany.
Sharma
Rita
R
Sippel
Claudia
C
Waegemann
Karin
K
Soll
Jürgen
J
Vothknecht
Ute C
UC
D000595:Amino Acid Sequence; D029681:Arabidopsis Proteins / Q000378:metabolism*; D002736:Chloroplasts / Q000201:enzymology* / Q000378:metabolism; D003600:Cytosol / Q000201:enzymology; D007527:Isoenzymes / Q000378:metabolism; D008969:Molecular Sequence Data; D005810:Multigene Family; D010766:Phosphorylation; D018515:Plant Leaves / Q000201:enzymology / Q000378:metabolism; D011498:Protein Precursors / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D021122:Protein Subunits / Q000378:metabolism; D021381:Protein Transport
D029681:Arabidopsis Proteins; D007527:Isoenzymes; D011498:Protein Precursors; D021122:Protein Subunits; D017346:Protein Serine-Threonine Kinases