Mechanism of self/nonself-discrimination in Brassica self-incompatibility

Kohji Murase,Go Suzuki,Yoshinobu Takada,Chiho Masaka,Sota Fujii,Seiji Takayama,Yoshitaka Moriwaki,Masao Watanabe,Tohru Terada,Ryoko Maesaki,Zen Kawabe,Toshio Hakoshima,Yoshinori Hirano,Kentaro Shimizu,Masaki Mishima,Koji Nagata,Xiao Liu,Tomoyuki Mori

doi:10.1038/s41467-020-18698-w

Abstract

Self-incompatibility (SI) is a breeding system that promotes cross-fertilization. In Brassica, pollen rejection is induced by a haplotype-specific interaction between pistil determinant SRK (S receptor kinase) and pollen determinant SP11 (S-locus Protein 11, also named SCR) from the S-locus. Although the structure of the B. rapa S9-SRK ectodomain (eSRK) and S9-SP11 complex has been determined, it remains unclear how SRK discriminates self- and nonself-SP11. Here, we uncover the detailed mechanism of self/nonself-discrimination in Brassica SI by determining the S8-eSRK–S8-SP11 crystal structure and performing molecular dynamics (MD) simulations. Comprehensive binding analysis of eSRK and SP11 structures reveals that the binding free energies are most stable for cognate eSRK–SP11 combinations. Residue-based contribution analysis suggests that the modes of eSRK–SP11 interactions differ between intra- and inter-subgroup (a group of phylogenetically neighboring haplotypes) combinations. Our data establish a model of self/nonself-discrimination in Brassica SI.

Highlights

Self-incompatibility (SI) is a breeding system that promotes cross-fertilization
To further understand the mechanism by which S receptor kinase (SRK) discriminates self- or nonself-S-locus Protein 11 (SP11) in Brassica SI, we tried to determine the structure of the B. rapa S8-eSRK–S8-SP1110,30 complex
Pull-down, isothermal calorimetry (ITC), gelfiltration, and chemical shift perturbation (CSP) analysis monitored by 1H-15N HSQC spectra revealed that S8-meSRK still strongly bound S8-SP11 (Fig. 1b–d; Supplementary Fig. 1c–e)

Summary

Introduction

Self-incompatibility (SI) is a breeding system that promotes cross-fertilization. In Brassica, pollen rejection is induced by a haplotype-specific interaction between pistil determinant SRK (S receptor kinase) and pollen determinant SP11 (S-locus Protein 11, named SCR) from the S-locus. Pistil factor SRK, localized on the plasma membrane of the papilla cell, recognizes its cognate pollen factor SP11 released from the pollen surface, triggering self-phosphorylation of the kinase domain[16,17,18,19], and transduces the SI signal to intracellular downstream effectors, resulting in pollen rejection[20,21,22,23,24,25] Both SRK and SP11 are highly polymorphic proteins among the haplotypes, with three hypervariable (HV I–III) regions in SRK; SP11 proteins have few similarities except for the signal sequences and cysteines forming the disulfide bonds[26,27]. Our comprehensive interaction analysis of modeled and known eSRK and SP11 structures by molecular dynamics (MD) simulations reveals the important features for understanding the mechanism of self/nonself-discrimination in Brassica SI

Methods

Results

Conclusion