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J. Biol. Chem., Vol. 282, Issue 46, 33845-33858, November 16, 2007

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Two SCA (Stigma/Style Cysteine-rich Adhesin) Isoforms Show Structural Differences That Correlate with Their Levels of in Vitro Pollen Tube Adhesion Activity^*

Keun Chae, Kangling Zhang^¶, Li Zhang^||, Dimitrios Morikis^**, Sun Tae Kim, Jean-Claude Mollet, Noelle de la Rosa, Kimberly Tan, and Elizabeth M. Lord¹

From the Center for Plant Cell Biology, Department of Botany and Plant Sciences, ^¶Mass Spectrometry Facility, ^||Department of Chemistry, and ^**Department of Bioengineering, University of California, Riverside, California 92521, the Environmental Biotechnology National Core Research Center, Gyeongsang National University, Jinju 660-701, Korea, and the Laboratoire de Glycobiologie et Transports chez les Végétaux, UMR CNRS 6037, IRFPM 23, Université de Rouen, 76821 Mont Saint-Aignan Cedex, France

Lily pollen tubes grow adhering to an extracellular matrix produced by the transmitting tract epidermis in a hollow style. SCA, a small (9.4 kDa), basic protein plus low esterified pectin from this extracellular matrix are involved in the pollen tube adhesion event. The mode of action for this adhesion event is unknown. We partially separated three SCA isoforms from the lily stigma in serial size exclusion column fractions (SCA1, 9370 Da; SCA2, 9384 Da; SCA3, 9484 Da). Peptide sequencing analysis allowed us to determine two amino acid variations in SCA3, compared with SCA1. For SCA2, however, there are more sequence variations yet to be identified. Our structural homology and molecular dynamics modeling results show that SCA isoforms have the plant nonspecific lipid transfer protein-like structure: a globular shape of the orthogonal 4-helix bundle architecture, four disulfide bonds, an internal hydrophobic and solvent-inaccessible cavity, and a long C-terminal tail. The Ala⁷¹ in SCA3, replacing the Gly⁷¹ in SCA1, has no predictable effect on structure. The Arg²⁶ in SCA3, replacing the Gly²⁶ in SCA1, is predicted to cause structural changes that result in a significantly reduced volume for the internal hydrophobic cavity in SCA3. The volume of the internal cavity fluctuates slightly during the molecular dynamics simulation, but overall, SCA1 displays a larger cavity than SCA3. SCA1 displays higher activity than SCA3 in the in vitro pollen tube adhesion assay. No differences were found between the two SCAs in a binding assay with pectin. The larger size of the hydrophobic cavity in SCA1 correlates with its higher adhesion activity.

Received for publication, May 15, 2007 , and in revised form, September 17, 2007.

^* This work was supported by National Science Foundation Grant IBM0420445 (to E. M. L.) and accomplished in partial fulfillment of a Ph.D. thesis for Keun Chae in the Cell, Molecular, and Developmental Biology Ph.D. program at the University of California, Riverside. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1-S8 and Tables S1-S3.

¹ To whom correspondence should be addressed: Center for Plant Cell Biology, Dept. of Botany and Plant Sciences, University of California, Riverside, CA 92521. Tel.: 951-827-4441; Fax: 951-827-4437; E-mail: lord{at}ucr.edu.

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