title: Structural modules for receptor dimerization in the S-locus receptor kinase extracellular domain.
abstract: 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.
authors.affiliation: Department of Plant Biology, Cornell Theory Center, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Sushma
authors.initials: S
authors.lastname: Naithani
authors.forename: Thanat
authors.initials: T
authors.lastname: Chookajorn
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Ripoll
authors.forename: June B
authors.initials: JB
authors.lastname: Nasrallah
mesh_terms: D000483:Alleles; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology*; D001937:Brassica / Q000201:enzymology*; D019281:Dimerization; D008024:Ligands; D008954:Models, Biological; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism*; D017510:Protein Folding; D011494:Protein Kinases / Q000737:chemistry* / Q000378:metabolism*; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000166:cytology; D040681:Structural Homology, Protein; D013329:Structure-Activity Relationship
chemical_list: D008024:Ligands; D010940:Plant Proteins; D011494:Protein Kinases; C079832:S-receptor kinase