title: Analysis of the complexity of protein kinases within the phloem sieve tube system. Characterization of Cucurbita maxima calmodulin-like domain protein kinase 1.
abstract: In angiosperms, functional, mature sieve elements lack nuclei, vacuoles, ribosomes, and most of the endomembrane network. In this study, the complexity, number, and nature of protein kinases within the phloem sap of Cucurbita maxima were investigated to test the hypothesis that the enucleate sieve tube system utilizes a simplified signal transduction network. Supporting evidence was obtained in that only five putative protein kinases (three calcium-independent and two calcium-dependent protein kinases) were detected within the phloem sap extracted from stem tissues. Biochemical methods were used to purify one such calcium-dependent protein kinase. The gene for this C. maxima calmodulin-like domain protein kinase 1 (CmCPK1), was cloned using peptide microsequences. A combination of mass spectrometry, peptide fingerprinting, and amino-terminal sequencing established that, in the phloem sap, CmCPK1 exists as an amino-terminally cleaved protein. A second highly homologous isoform, CmCPK2, was identified, but although transcripts could be detected in the companion cells, peptide fingerprint analysis suggested that CmCPK2 does not enter the phloem sap. Potential substrates for CmCPK1, within the phloem sap, were also detected using an on-membrane phosphorylation assay. Entry of CmCPK1 into sieve elements via plasmodesmata and the potential roles played by these phloem protein kinases are discussed.
authors.affiliation: Section of Plant Biology, Division of Biological Sciences, University of California, Davis, California 95616, USA.
authors.forename: Byung-Chun
authors.initials: BC
authors.lastname: Yoo
authors.forename: Jung-Youn
authors.initials: JY
authors.lastname: Lee
authors.forename: William J
authors.initials: WJ
authors.lastname: Lucas
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000737:chemistry* / Q000378:metabolism; D002147:Calmodulin / Q000737:chemistry*; D017124:Conserved Sequence; D028464:Cucurbita / Q000201:enzymology*; D007527:Isoenzymes / Q000737:chemistry; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry; D010766:Phosphorylation; D018514:Plant Structures / Q000201:enzymology; D011494:Protein Kinases / Q000737:chemistry* / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D002147:Calmodulin; D007527:Isoenzymes; D010446:Peptide Fragments; D011494:Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases