GPI anchor remodeling by the plant PGAP1 ortholog HLD1 is essential for Papaver self-incompatibility

In eukaryotes, glycosylphosphatidylinositol anchored proteins (GPI-APs) are tethered to the outer leaflet of the plasma membrane where they function as key regulators of a plethora of biological processes. Self-incompatibility (SI) plays a pivotal role regulating fertilization in higher plants through recognition and rejection of self pollen. Here we used Arabidopsis thaliana lines engineered to be self-incompatible by expression of Papaver SI determinants for an SI suppressor screen. We identify HLD1, an ortholog of human GPI-inositol deacylase PGAP1, whose mutation completely abolishes the SI response. We show that HLD1 functions as a GPI-inositol deacylase and that this GPI-remodeling activity is essential for SI. Using GFP-SKU5 as a representative GPI-AP, we show that HLD1 mutation does not affect GPI-AP production and targeting, but alters the configuration of mature GPI-APs. This prevents GPI-AP release from the plasma membrane, suggesting that this process plays a critical role in the regulation of SI. Our data not only identify GPI anchoring as a new pathway of SI providing new directions to investigate SI mechanisms, but identifies for the first time a function for GPI-AP remodeling by inositol deacylation in plants. One sentence summaryThe Papaver self-incompatibility response requires GPI-anchor modification by HLD1, an ortholog of the mammalian PGAP1.