Journal Article

Structural modules for receptor dimerization in the S-locus receptor kinase extracellular domain

Ripoll, Daniel R, Nasrallah, June B, Naithani, Sushma, Chookajorn, Thanat

2007

Overview

The highly polymorphic S-locus receptor kinase (SRK) is the stigma determinant of specificity in the self-incompatibility response of the Brassicaceae. SRK spans the plasma membrane of stigma epidermal cells, and it is activated in an allele-specific manner on binding of its extracellular region (eSRK) to its cognate pollen coat-localized S-locus cysteine-rich (SCR) ligand. SRK, like several other receptor kinases, forms dimers in the absence of ligand. To identify domains in SRK that mediate ligand-independent dimerization, we assayed eSRK for self-interaction in yeast. We show that SRK dimerization is mediated by two regions in eSRK, primarily by a C-terminal region inferred by homology modeling/fold recognition techniques to assume a PAN_APPLE-like structure, and secondarily by a region containing a signature sequence of the S-domain gene family, which might assume an EGF-like structure. We also show that eSRK exhibits a marked preference for homodimerization over heterodimerization with other eSRK variants and that this preference is mediated by a small, highly variable region within the PAN_APPLE domain. Thus, the extensive polymorphism exhibited by the eSRK not only determines differential affinity toward the SCR ligand, as has been assumed thus far, but also underlies a previously unrecognized allelic specificity in SRK dimerization. We propose that preference for SRK homodimerization explains the codominance exhibited by a majority of SRKs in the typically heterozygous stigmas of self-incompatible plants, whereas an increased propensity for heterodimerization combined with reduced affinity of heterodimers for cognate SCRs might underlie the dominant-recessive or mutual weakening relationships exhibited by some SRK allelic pairs.

• Publisher: National Academy of Sciences,National Acad Sciences
• Subject: Agglutination / Alleles / Amino Acid Sequence / Amino acids / Arabidopsis / Arabidopsis - enzymology / Arabidopsis lyrata / Biochemistry / Biological Sciences / Brassica - enzymology / Brassica oleracea / Brassicaceae / Cell aggregates / Cells / Dimerization / Disulfides / Kinases / Ligands / membrane proteins / Models, Biological / Models, Molecular / Molecular Sequence Data / Molecules / Plant Proteins - chemistry / Plant Proteins - metabolism / Plasma / plasma membrane / Pollen / protein conformation / Protein Folding / protein kinases / Protein Kinases - chemistry / Protein Kinases - metabolism / protein structure / Protein Structure, Secondary / Protein Structure, Tertiary / Proteins / Receptors / S-receptor kinase / Saccharomyces cerevisiae - cytology / stigma / Structural Homology, Protein / Structure-Activity Relationship / Three dimensional modeling / Yeasts