title: Ricinus communis cyclophilin: functional characterisation of a sieve tube protein involved in protein folding.
abstract: The phloem translocation stream of the angiosperms contains a special population of proteins and RNA molecules which appear to be produced in the companion cells prior to being transported into the sieve tube system through the interconnecting plasmodesmata. During this process, these non-cell-autonomous proteins are thought to undergo partial unfolding. Recent mass spectroscopy studies identified peptidyl-prolyl cis-trans isomerase (PPIases) as potential molecular chaperones functioning in the phloem translocation stream (Giavalisco et al. 2006). In the present study, we describe the cloning and characterisation of a castor bean phloem cyclophilin, RcCYP1 that has high peptidyl-prolyl cis-trans isomerase activity. Equivalent enzymatic activity was detected with phloem sap or purified recombinant (His)(6)-tagged RcCYP1. Mass spectrometry analysis of proteolytic peptides, derived from a 22 kDa band in HPLC-fractionated phloem sap, immunolocalisation studies and Western analysis of proteins extracted from castor bean tissues/organs indicated that RcCYP1 is an abundant protein in the companion cell-sieve element complex. Microinjection experiments established that purified recombinant (His)(6)-RcCYP1 can interact with plasmodesmata to both induce an increase in size exclusion limit and mediate its own cell-to-cell trafficking. Collectively, these findings support the hypothesis that RcCYP1 plays a role in the refolding of non-cell-autonomous proteins after their entry into the phloem translocation stream.
authors.affiliation: Pflanzenphysiologie, Universität Bayreuth, 95440 Bayreuth, Germany.
authors.forename: Maren
authors.initials: M
authors.lastname: Gottschalk
authors.forename: Elmar
authors.initials: E
authors.lastname: Dolgener
authors.forename: Beatriz
authors.initials: B
authors.lastname: Xoconostle-Cázares
authors.forename: William J
authors.initials: WJ
authors.lastname: Lucas
authors.forename: Ewald
authors.initials: E
authors.lastname: Komor
authors.forename: Christian
authors.initials: C
authors.lastname: Schobert
mesh_terms: D000595:Amino Acid Sequence; D002367:Ricinus communis / Q000378:metabolism*; D021983:Cyclophilins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression; D015723:Gene Library; D008969:Molecular Sequence Data; D019696:Peptidylprolyl Isomerase / Q000235:genetics*; D052585:Phloem / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism*; D011487:Protein Conformation; D017510:Protein Folding; D012278:Ricinus / Q000201:enzymology*; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA
chemical_list: D010940:Plant Proteins; D021983:Cyclophilins; D019696:Peptidylprolyl Isomerase