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, Tae Kim Sun, Jean Claude Mollet, Noelle De La Rosa, Kimberly Tan, Elizabeth M. Lord

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Abstract

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 non-specific 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.

Original languageEnglish (US)
Pages (from-to)33845-33858
Number of pages14
JournalJournal of Biological Chemistry
Volume282
Issue number46
DOIs
StatePublished - Nov 16 2007
Externally publishedYes

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ASJC Scopus subject areas

  • Biochemistry

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