title: Functional analysis of Brassica napus phloem protein and ribonucleoprotein complexes.
abstract: Phloem sap contains a large number of macromolecules, including proteins and RNAs from different classes. Proteome analyses of phloem samples from different plant species under denaturing conditions identified hundreds of proteins potentially involved in diverse processes. Surprisingly, these studies also found a significant number of ribosomal and proteasomal proteins. This led to the suggestion that active ribosome and proteasome complexes might be present in the phloem, challenging the paradigm that protein synthesis and turnover are absent from the enucleate sieve elements of angiosperms. However, the existence of such complexes has as yet not been demonstrated. In this study we used three-dimensional gel electrophoresis to separate several protein complexes from native phloem sap from Brassica napus. Matrix-assisted laser desorption ionization-time of flight MS analyses identified more than 100 proteins in the three major protein-containing complexes. All three complexes contained proteins belonging to different ribosomal fragments and blue native northern blot confirmed the existence of ribonucleoprotein complexes. In addition, one complex contained proteasome components and further functional analyses confirmed activity of a proteasomal degradation pathway and showed a large number of ubiquitinated phloem proteins. Our results suggest specialized roles for ubiquitin modification and proteasome-mediated degradation in the phloem.
authors.affiliation: Molecular Plant Genetics, University Hamburg, Biocenter Klein Flottbek, Ohnhorststr. 18, Hamburg, 22609, Germany.
authors.forename: Anna
authors.initials: A
authors.lastname: Ostendorp
authors.affiliation: Molecular Plant Genetics, University Hamburg, Biocenter Klein Flottbek, Ohnhorststr. 18, Hamburg, 22609, Germany.
authors.forename: Steffen
authors.initials: S
authors.lastname: Pahlow
authors.affiliation: Molecular Plant Genetics, University Hamburg, Biocenter Klein Flottbek, Ohnhorststr. 18, Hamburg, 22609, Germany.
authors.forename: Lena
authors.initials: L
authors.lastname: Krüßel
authors.affiliation: Molecular Plant Genetics, University Hamburg, Biocenter Klein Flottbek, Ohnhorststr. 18, Hamburg, 22609, Germany.
authors.forename: Patrizia
authors.initials: P
authors.lastname: Hanhart
authors.affiliation: Molecular Plant Genetics, University Hamburg, Biocenter Klein Flottbek, Ohnhorststr. 18, Hamburg, 22609, Germany.
authors.forename: Marcel Y
authors.initials: MY
authors.lastname: Garbe
authors.affiliation: Molecular Plant Genetics, University Hamburg, Biocenter Klein Flottbek, Ohnhorststr. 18, Hamburg, 22609, Germany.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Deke
authors.affiliation: Max Planck Institute of Molecular Plant Physiology, am Mühlenberg 1, Potsdam, 14476, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Giavalisco
authors.affiliation: Molecular Plant Genetics, University Hamburg, Biocenter Klein Flottbek, Ohnhorststr. 18, Hamburg, 22609, Germany.
authors.forename: Julia
authors.initials: J
authors.lastname: Kehr
mesh_terms: D029688:Brassica napus / Q000378:metabolism*; D008970:Molecular Weight; D046912:Multiprotein Complexes / Q000378:metabolism*; D052585:Phloem / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D059748:Proteolysis; D012261:Ribonucleoproteins / Q000378:metabolism*; D012270:Ribosomes / Q000378:metabolism; D057149:Ubiquitinated Proteins / Q000378:metabolism
chemical_list: D046912:Multiprotein Complexes; D010940:Plant Proteins; D012261:Ribonucleoproteins; D057149:Ubiquitinated Proteins; D046988:Proteasome Endopeptidase Complex
keywords: 
  Brassica napus
  ; blue native PAGE; mass spectrometry; phloem; proteasome-mediated protein degradation; protein complex; ribonucleoprotein complex