Greasing endocytosis in plants - understanding the role of S-acylation in receptor kinase function and internalisation

Receptor kinases are proteins that act as the primary means by which plants perceive physical signals in their extracellular environment. This includes pathogenic and beneficial microbes, the state of the plant's own cell wall and various plant hormones. As a result, receptor kinases are critical to controlling how plants balance resource allocation during growth, trading resistance to pathogens against seed yield or vegetative productivity. Given their powerful ability to control and balance how plants respond to their environment it is no surprise that they are very tightly regulated and controlled. Part of this regulation involves ensuring that receptor kinases that have perceived a signal are removed from the plant surface after a period of time to allow for measured and graded environmental monitoring and integration of competing perception events, rather than a single stimulation event overwhelming the plant. This is achieved through a process known as endocytosis whereby proteins on the outer edge of the cell are internalised and destroyed. This requires a number of events including the "tagging" of the proteins with another protein known as ubiquitin. This marks the tagged protein for endocytosis. We recently discovered that another modification of receptor kinases affects endocytosis. This modification, known as S-acylation, involves adding fatty acids to receptor kinases once they have perceived a stimulus. We found that S-acylation is essential for correct action by a receptor and is necessary for appropriate endocytosis. Loss of S-acylation actually causes more rapid endocytosis through an unknown route and, as a result, leads to poor signal strength and ineffective response to the stimulus. In this work we want to understand why loss of S-acylation causes more rapid endocytosis, what dictates that receptor S-acylation will occur and the route of internalisation taken by receptors that can't be S-acylated. This will help us to understand how receptor kinase signalling is regulated to deliver appropriate responses to the external environment. Receptor kinases are the frontline of how plants perceive their world; understanding how receptor kinases function could therefore be key towards supporting breeding and transgenic approaches to improve pathogen resistance, plant growth and yield or environmental or climate challenge such as drought, flooding, high or low temperature or salinity.

Receptor kinases act as the main receptors for plants perception of physical signals in their extracellular environment. This includes pathogenic and beneficial microbes, the state of the plant's own cell wall and various plant hormones. As a result, receptor kinases are critical to controlling how plants balance resource allocation during growth, trading resistance to pathogens against seed or vegetative productivity. Given their powerful ability to control and balance how plants respond to their environment it is no surprise that their action is very tightly regulated and controlled. Following activation, receptor kinases undergo UBC35/36 mediated K63-linked polyubiquitination to promote clathrin mediated endocytosis, supress signalling and prevent overstimulation through the pathway. We recently demonstrated that receptor kinases, specifically FLS2 and EFR, are post-translationally modified by fatty acids (S-acylation) and that, unexpectedly, S-acylation appears to delay receptor endocytosis. We hypothesise that S-acylation either impedes the polyubiquitination action of UBC35/36 against FLS2 or delays clathrin mediated endocytosis of polyubiquitinated FLS2. Using a combination of biochemistry and cell biology we aim to determine how S-acylation and K63-linked polyubiquitination interact to dictate activated FLS2 signal duration and endocytosis rates. We have also observed that FLS2 S-acylation state falls over time; we will investigate whether this is regulatory or catabolic in nature to define potential post-endocytic regulation of FLS2 function by de-S-acylation. These data will provide key insights into how receptor kinase signalling is amplified, sustained, and attenuated in all plant species. Understanding these fundamental details of how receptor kinases function could therefore be key towards supporting breeding and transgenic approaches to improve pathogen resistance, boost plant growth and yield or mitigate against environmental or climatic challenges.