Two SCA (stigma/style cysteine-rich adhesin) isoforms show structural differences that correlate with their levels of in vitro pollen tube adhesion activity.
Lily pollen tubes grow adhering to an extracellular matrix produced by the transmitting tract epidermis in a hollow style. SCA, a small ( approximately 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 Ala(71) in SCA3, replacing the Gly(71) in SCA1, has no predictable effect on structure. The Arg(26) in SCA3, replacing the Gly(26) 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.
Chae
Keun
K
Center for Plant Cell Biology, University of California, Riverside, California, 92521; Department of Botany and Plant Sciences, University of California, Riverside, California, 92521.
Zhang
Kangling
K
Mass Spectrometry Facility, University of California, Riverside, California, 92521.
Zhang
Li
L
Department of Chemistry, University of California, Riverside, California, 92521.
Morikis
Dimitrios
D
Department of Bioengineering, University of California, Riverside, California 92521.
Kim
Sun Tae
ST
Environmental Biotechnology National Core Research Center, Gyeongsang National University, Jinju 660-701, Korea.
Mollet
Jean-Claude
JC
Laboratoire de Glycobiologie et Transports chez les Végétaux, UMR CNRS 6037, IRFPM 23, Université de Rouen, 76821 Mont Saint-Aignan Cedex, France.
de la Rosa
Noelle
N
Department of Botany and Plant Sciences, University of California, Riverside, California, 92521.
Tan
Kimberly
K
Department of Botany and Plant Sciences, University of California, Riverside, California, 92521.
Lord
Elizabeth M
EM
Center for Plant Cell Biology, University of California, Riverside, California, 92521; Department of Botany and Plant Sciences, University of California, Riverside, California, 92521. Electronic address: lord@ucr.edu.
D000409:Alanine / Q000737:chemistry; D000595:Amino Acid Sequence; D002448:Cell Adhesion; D005109:Extracellular Matrix / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D027762:Lilium / Q000378:metabolism*; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D010368:Pectins / Q000737:chemistry; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism; D011058:Pollen / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D020033:Protein Isoforms; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid
D010940:Plant Proteins; D020033:Protein Isoforms; D010368:Pectins; D000409:Alanine