Control of PHYTOCHROME KINASE SUBSTRATE subcellular localization and biological activity by protein S-acylation

Abstract

PHYTOCHROME KINASE SUBSTRATE (PKS) proteins are involved in light-regulated growth orientation responses. 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 the phototropins at the plasma membrane. Identifying evolutionarily 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 6 motifs (A to F from the N- to the C-terminus). While motif D is also found in BIG GRAIN proteins the remining domains are PKS specific. We provide evidence that motif C is S-acylated on highly conserved cysteines, which mediates PKS protein association with the plasma membrane. This motif is also required for PKS4-mediated phototropism and control of hypocotyl gravitropism in the light. Finally, our data suggests that the mode of PKS4 plasma membrane association is important for its biological activity. Our work identifies the mode of plasma membrane association of PKS proteins and strongly suggests that this is their site of action to modulate environmentally regulated organ positioning.