Direct ligand–receptor complex interaction controls Brassica self-incompatibility

Abstract

Many higher plants have evolved self-incompatibility mechanisms to prevent self-fertilization. In Brassica self-incompatibility, recognition between pollen and the stigma is controlled by the S locus, which contains three highly polymorphic genes: S-receptor kinase (SRK), S-locus protein 11 (SP11) (also called S-locus cysteine-rich protein; SCR) and S-locus glycoprotein (SLG). SRK encodes a membrane-spanning serine/threonine kinase that determines the S-haplotype specificity of the stigma, and SP11 encodes a small cysteine-rich protein that determines the S-haplotype specificity of pollen. SP11 is localized in the pollen coat. It is thought that, during self-pollination, SP11 is secreted from the pollen coat and interacts with its cognate SRK in the papilla cell of the stigma to elicit the self-incompatibility response. SLG is a secreted stigma protein that is highly homologous to the SRK extracellular domain. Although it is not required for S-haplotype specificity of the stigma, SLG enhances the self-incompatibility response; however, how this is accomplished remains controversial. Here we show that a single form of SP11 of the S8 haplotype (S8-SP11) stabilized with four intramolecular disulphide bonds specifically binds the stigma membrane of the S8 haplotype to induce autophosphorylation of SRK8, and that SRK8 and SLG8 together form a high-affinity receptor complex for S8-SP11 on the stigma membrane.

A Cysteine-Rich Extracellular Protein, LAT52, Interacts with the Extracellular Domain of the Pollen Receptor Kinase LePRK2[W]

“…The first group comprises diverse Cys-rich proteins (molecular mass ranging from 6 to 16 kD). This group was especially interesting because another small Cys-rich protein, SCR, found on the pollen coat in Brassica species, was shown recently to be the ligand for the stigma-expressed S-locus receptor kinase SRK (Schopfer et al, 1999;Kachroo et al, 2001;Takayama et al, 2001). However, SRK is not an LRR kinase, and the arrangement of Cys residues is not conserved between SCR and the Cys-rich proteins we identified from the yeast two-hybrid screen.…”

Structural basis for specific self-incompatibility response in Brassica

“…In the Brassicaceae family, SI is mediated by variant haplotypes of a single highly polymorphic genetic locus, termed the S locus [2], which generally contains three highly polymorphic genes, the stigma-expressed S-locus receptor kinase (SRK), the pollen-expressed S-locus cysteine-rich protein (SCR; or S-locus protein 11) and the S-locus glycoprotein (SLG) [3][4][5][6][7]. Genetic and biochemical studies established SRK and SCR as the sole determinants of SI specificity, and SRK as the receptor for SCR, which allows the stigma to discriminate between "self" and "non-self" pollen in the SI response [4,[8][9][10][11]. Because an SCR protein will only bind and activate the SRK variant encoded in the same S-locus haplotype, the SRK and SCR proteins must co-evolve to maintain their interaction.…”

Self-Incompatibility in the Brassicaceae

“…A binding experiment using 125 I-labeled-S 8 -SP11 suggested that it strongly Fig. 21.1 Self-and cross-pollination in Brassica rapa binds to the stigmatic membrane of S 8 -haplotype ( K d = 0.7 nM) but not of the S -haplotype (Takayama et al 2001 ). Cross-linking and immunological analyses suggested that 125 I-labeled-S 8 -SP11 directly binds to S 8 -SRK and a 60-kDa protein in the stigmatic membrane of S 8 -haplotype (Takayama et al 2001 ).…”

“…Upon pollination, SP11/SCR penetrates the papilla cell wall and binds SRK in an S -haplotype-specifi c manner. This binding induces the autophosphorylation of SRK, triggering a signaling cascade that results in the rejection of self-pollen (Takayama et al 2001 ;Takayama and Isogai 2005 ;Iwano and Takayama 2012 ) (Fig. 21.3 ).…”

“…21.1 Self-and cross-pollination in Brassica rapa binds to the stigmatic membrane of S 8 -haplotype ( K d = 0.7 nM) but not of the S -haplotype (Takayama et al 2001 ). Cross-linking and immunological analyses suggested that 125 I-labeled-S 8 -SP11 directly binds to S 8 -SRK and a 60-kDa protein in the stigmatic membrane of S 8 -haplotype (Takayama et al 2001 ). Affi nity purifi cation and LC-MS/MS analysis of SP11-binding stigmatic proteins have revealed that the 60-kDa protein is a truncated form of SRK (tSRK) containing the extracellular, transmembrane, and part of the intracellular juxtamembrane domains ( Shimosato et al 2007 ).…”