Protein S-acylation controls the subcellular localization and biological activity of PHYTOCHROME KINASE SUBSTRATE.

PHYTOCHROME KINASE SUBSTRATE (PKS) proteins are involved in light-modulated changes in growth orientation. They act downstream of phytochromes to control hypocotyl gravitropism in the light and act early in phototropin signaling. Despite their importance for plant development, little is known about their molecular mode of action, except that they belong to a protein complex comprising phototropins at the plasma membrane. Identifying evolutionary conservation is one approach to reveal biologically important protein motifs. Here, we show that PKS sequences are restricted to seed plants and that these proteins share six motifs (A to F from the N to the C terminus). Motifs A and D are also present in BIG GRAIN, while the remaining four are specific to PKSs. We provide evidence that motif C is S-acylated on highly conserved cysteines, which mediates the association of PKS proteins with the plasma membrane. Motif C is also required for PKS4-mediated phototropism and light-regulated hypocotyl gravitropism. Finally, our data suggest that the mode of PKS4 association with the plasma membrane is important for its biological activity. Our work therefore identifies conserved cysteines contributing to plasma membrane association of PKS proteins and strongly suggests that this is their site of action to modulate environmentally regulated organ positioning.