title: Molecular characterization of a phloem-specific gene encoding the filament protein, phloem protein 1 (PP1), from Cucurbita maxima.
abstract: Sieve elements in the phloem of most angiosperms contain proteinaceous filaments and aggregates called P-protein. In the genus Cucurbita, these filaments are composed of two major proteins: PP1, the phloem filament protein, and PP2, the phloem lactin. The gene encoding the phloem filament protein in pumpkin (Cucurbita maxima Duch.) has been isolated and characterized. Nucleotide sequence analysis of the reconstructed gene gPP1 revealed a continuous 2430 bp protein coding sequence, with no introns, encoding an 809 amino acid polypeptide. The deduced polypeptide had characteristics of PP1 and contained a 15 amino acid sequence determined by N-terminal peptide sequence analysis of PP1. The sequence of PP1 was highly repetitive with four 200 amino acid sequence domains containing structural motifs in common with cysteine proteinase inhibitors. Expression of the PP1 gene was detected in roots, hypocotyls, cotyledons, stems, and leaves of pumpkin plants. PP1 and its mRNA accumulated in pumpkin hypocotyls during the period of rapid hypocotyl elongation after which mRNA levels declined, while protein levels remained elevated. PP1 was immunolocalized in slime plugs and P-protein bodies in sieve elements of the phloem. Occasionally, PP1 was detected in companion cells. PP1 mRNA was localized by in situ hybridization in companion cells at early stages of vascular differentiation. The developmental accumulation and localization of PP1 and its mRNA paralleled the phloem lactin, further suggesting an interaction between these phloem-specific proteins.
authors.affiliation: Department of Plant Sciences, University of Arizona, Tucson 86712, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Clark
authors.forename: K R
authors.initials: KR
authors.lastname: Jacobsen
authors.forename: D E
authors.initials: DE
authors.lastname: Bostwick
authors.forename: J M
authors.initials: JM
authors.lastname: Dannenhoffer
authors.forename: M I
authors.initials: MI
authors.lastname: Skaggs
authors.forename: G A
authors.initials: GA
authors.lastname: Thompson
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis; D001483:Base Sequence; D016384:Consensus Sequence; D017124:Conserved Sequence; D015853:Cysteine Proteinase Inhibitors / Q000737:chemistry; D018744:DNA, Plant / Q000737:chemistry / Q000302:isolation & purification; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D020930:MAP Kinase Kinase Kinases; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics*; D014176:Protein Biosynthesis; D011494:Protein Kinases / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics; D017433:Protein Structure, Secondary; D012333:RNA, Messenger / Q000096:biosynthesis; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014158:Transcription, Genetic; D014675:Vegetables / Q000235:genetics / Q000378:metabolism*
chemical_list: C108212:CPC7 protein, Cucurbita maxima; D015853:Cysteine Proteinase Inhibitors; D018744:DNA, Plant; D010940:Plant Proteins; D012333:RNA, Messenger; D011494:Protein Kinases; C108211:ANP1 kinase; D020930:MAP Kinase Kinase Kinases