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CELL BIOLOGY AND SIGNAL TRANSDUCTION

An Ankyrin Repeat-Containing Protein, Characterized as a Ubiquitin Ligase, Is Closely Associated with Membrane-Enclosed Organelles and Required for Pollen Germination and Pollen Tube Growth in Lily^1,[W]

Protein Science Laboratory of the Ministry of Education, Department of Biological Sciences and Biotechnologies, Tsinghua University, Beijing 100084, China (J.H., F.C., M.S., S.Y., J.P., Y.L.); Dipartimento di Scienze Ambientali, Università di Siena, via Mattioli 4, 53100 Siena, Italy (C.D.C., A.A., M.C.); and Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200031, China (H.S., C.W.)

Exhibiting rapid polarized growth, the pollen tube delivers the male gametes into the ovule for fertilization in higher plants. To get an overall picture of gene expression during pollen germination and pollen tube growth, we profiled the transcription patterns of 1,536 pollen cDNAs from lily (Lilium longiflorum) by microarray. Among those that exhibited significant differential expression, a cDNA named lily ankyrin repeat-containing protein (LlANK) was thoroughly studied. The full-length LlANK cDNA sequence predicts a protein containing five tandem ankyrin repeats and a RING zinc-finger domain. The LlANK protein possesses ubiquitin ligase activity in vitro. RNA blots demonstrated that LlANK transcript is present in mature pollen and its level, interestingly contrary to most pollen mRNAs, up-regulated significantly during pollen germination and pollen tube growth. When fused with green fluorescent protein and transiently expressed in pollen, LlANK was found dominantly associated with membrane-enclosed organelles as well as the generative cell. Overexpression of LlANK, however, led to abnormal growth of the pollen tube. On the other hand, transient silencing of LlANK impaired pollen germination and tube growth. Taken together, these results showed that LlANK is a ubiquitin ligase associated with membrane-enclosed organelles and required for polarized pollen tube growth.

² These authors contributed equally to the paper.

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: Yiqin Li (liyq{at}mail.tsinghua.edu.cn).

^[W] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.074922.

^* Corresponding author; e-mail liyq{at}mail.tsinghua.edu.cn; fax 86–10–62772678.

Received November 30, 2005; returned for revision January 4, 2006; accepted January 19, 2006.

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