Molecule of the Month: ZAR1 Resistosome

Like humans and all other organisms, plants can become infected by pathogens and stricken with disease. This is a serious concern for agriculture and food production, as diseases can wipe out entire populations of crops. Fortunately, plants already come equipped with their own set of immune defenses to protect themselves against such outside threats. However, unlike humans which have both adaptive and innate immunities, plants only have innate immunity. The innate immune system relies on receptors on the cell surface as well as inside the cell that detect certain molecular patterns and effector molecules associated with pathogens, respectively. When this detection event occurs, it will trigger a cascade of events that ultimately lead to an immune response to fend off the pathogen and maintain survival. Here we explore one of these pathways in Arabidopsis thaliana, a common weed plant that is widely used by researchers as a model organism, which culminates in the formation of a large molecular machine known as the resistosome.

As with any immune response, the resistosome formation pathway begins with the infection itself. Specifically, the pathogenic bacteria Xanthomonas campestris—a major culprit of “black rot” in leaves—initiates the offensive by infecting a plant cell with the uridylyltransferase AvrAC. Uridylyltransferases are a group of enzymes that—as their name implies—“transfer” a “uridylyl” nucleotide onto a target protein or nucleic acid chain, generally by attaching it to an amino acid with an available hydroxyl group such as a threonine or serine. Once inside the plant cell, AvrAC starts its attack by adding uridyl groups to regulatory kinase enzymes, which inhibits immune signaling pathways and thereby leads to increased virulence. Cleverly, however, the plant cell fights back by baiting it with a decoy protein called PBL2. When PBL2 is uridylylated, instead of shutting down the immune response, it begins the process of building the resistosome, as shown in the “Exploring the Structure” section below via an interactive JSmol visualization.

As observed in the top view of the resistosome (shown here from PDB ID 6j5t), the fully-formed assembly exhibits a ring-like structure and is composed of five copies of three different proteins. On the outer edges of the ring are the uridylylated PBL2 subunits (dark blue), each of which is bound toward the center to another subunit called RSK1 (a pseudokinase; turquoise). On the inside of the ring sits an immune-receptor subunit known as ZAR1 (green), and serves as the connecting point for the five complexes to join together. Notably, at the very center of the resistosome, the ends of the ZAR1 subunits (yellow) form a funnel-like protrusion on one side of the assembly, as depicted in the side view. This funnel is found to carry out the critical protective function of the ZAR1 resistosome by inserting itself into the cell membrane to form a pore and lead to a localized programmed cell death termed the “hypersensitive response,” thus sacrificing the infected cell for the good of the whole. Such an insertion mechanism can be envisioned given the relatively hydrophobic exterior of the funnel which would interact with the hydrophobic interior of the plasma membrane, as well as the funnel’s hollow hydrophilic interior which would be conducive to the cytosolic environment.

The computed structure model of AvrAC, shown here from AF-Q4UWF4-F1, is composed of two basic parts: a kinase-binding domain (in red) and a long, disordered tail (in white). The kinase-binding region not only facilitates the association of AvrAC with PBL2 (the decoy protein), but it is also responsible for carrying out the uridylylation activity of AvrAC. This activity is imparted by a small motif called a “Fic” domain (highlighted in dark red) which is capable of mediating nucleotide binding and transfer. Importantly, it has been found that AvrAC must uridylylate two sites on PBL2—one serine and one threonine—in order to trigger the cascade of events in the resistosome formation pathway.