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CELL BIOLOGY AND SIGNAL TRANSDUCTION

An Early Nodulin-Like Protein Accumulates in the Sieve Element Plasma Membrane of Arabidopsis^1,[OA]

Department of Applied Science, University of Arkansas, Little Rock, Arkansas 72204–1099 (J.A.K., G.A.T.); Department of Biological Sciences, University of Iowa, Iowa City, Iowa 52242 (Q.W., R.D.S.); and Department of Plant Biology, Faculty of Life Sciences, University of Copenhagen, DK–1871 Frederiksberg C, Denmark (A.S.)

Membrane proteins within the sieve element-companion cell complex have essential roles in the physiological functioning of the phloem. The monoclonal antibody line RS6, selected from hybridomas raised against sieve elements isolated from California shield leaf (Streptanthus tortuosus; Brassicaceae) tissue cultures, recognizes an antigen in the Arabidopsis (Arabidopsis thaliana) ecotype Columbia that is associated specifically with the plasma membrane of sieve elements, but not companion cells, and accumulates at the earliest stages of sieve element differentiation. The identity of the RS6 antigen was revealed by reverse transcription-PCR of Arabidopsis leaf RNA using degenerate primers to be an early nodulin (ENOD)-like protein that is encoded by the expressed gene At3g20570. Arabidopsis ENOD-like proteins are encoded by a multigene family composed of several types of structurally related phytocyanins that have a similar overall domain structure of an amino-terminal signal peptide, plastocyanin-like copper-binding domain, proline/serine-rich domain, and carboxy-terminal hydrophobic domain. The amino- and carboxy-terminal domains of the 21.5-kD sieve element-specific ENOD are posttranslationally cleaved from the precursor protein, resulting in a mature peptide of approximately 15 kD that is attached to the sieve element plasma membrane via a carboxy-terminal glycosylphosphatidylinositol membrane anchor. Many of the Arabidopsis ENOD-like proteins accumulate in gametophytic tissues, whereas in both floral and vegetative tissues, the sieve element-specific ENOD is expressed only within the phloem. Members of the ENOD subfamily of the cupredoxin superfamily do not appear to bind copper and have unknown functions. Phenotypic analysis of homozygous T-DNA insertion mutants for the gene At3g20570 shows minimal alteration in vegetative growth but a significant reduction in the overall reproductive potential.

² Present address: Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO 63167.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Gary A. Thompson (gathompson{at}ualr.edu).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.092296

^* Corresponding author; e-mail gathompson{at}ualr.edu; fax 501–569–8020.

Received October 30, 2006; accepted January 31, 2007; published February 9, 2007.

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