The RNAi pathway (covered before here) is a major immune mechanism in plants and invertebrates, but it can be suppressed by the very viruses it is trying to stop. The virus produces an RNA Silencing Suppressor (RSS), a protein that binds or degrades proteins in the RNAi pathway, giving the virus free reign to replicate as it pleases. This has the potential to create a fast-moving arms race: host suppresses virus using RNAi, so virus suppresses host suppression mechanism, so host suppresses the viral suppressor of host suppression…
Only the first two steps in this cycle have been found so far. The RNAi machinery has been extensively documented, and many viral RSSs have been catalogued. However, a recent paper in PNAS by Nakahara et al may have found the first instance of the third step in the cycle: a host suppressor of RSSs.
The authors focused on rgs-Cam, a tobacco plant protein that has previously been observed to interact with viral proteins (Anandalakshmi et al, 2000). They found that it binds RSSs with greater affinity if they have an arginine-rich domain. This domain is also what the RSS uses to bind small interfering RNAs (a constituent of the RNAi pathway), meaning that rgs-Cam may be specifically targeting only those proteins that suppress RNAi.
To test its effect on the activity of RSSs, the authors depleted rgs-Cam in tobacco (ironically by using RNAi to knock it down). This led to increased suppression of RNAi by two RSSs (2b and HC-Pro). They then created transgenic plants with either increased or decreased levels of rgs-Cam. They found that those with more rgs-Cam had reduced RSS activity and were less susceptible to viral attack, whereas those with less rgs-Cam had increased RSS activity and were more susceptible to attack by viruses.
So it looked like rgs-Cam suppressed RSSs, and therefore restores the RNAi response. But how? The authors inhibited different cellular pathways and found that when the autophagy pathway was inhibited, the levels of rgs-Cam and RSSs both increased. From this they concluded that once rgs-Cam binds an RSS, both are broken down by autophagy-like protein degradation (ALPD).
Interestingly, the authors highlight previous work that reported an inhibition of the RNAi mechanism by high levels of rgs-Cam. This causes them to speculate that rgs-Cam may be triggered in emergencies only. When the normal RNAi pathway can deal with the virus, rgs-Cam levels are kept low; however, when the virus gets out of hand, rgs-Cam is upregulated (or de-inhibited, the authors are admirably honest about the remaining ambiguity in the exact mechanism).
Role on the discovery of the first VSHSRSS – Viral Suppressor of Host Suppression of RNA Silencing Suppressor!
Nakahara, K.S. et al (2012) Tobacco calmodulin-like protein provides secondary defense by binding to and directing degradation of virus RNA silencing suppresors. PNAS Early Edition: 1-6.
Anandalakshmi, R. et al (2000) A calmodulin-related protein that suppresses post-transcriptional gene silencing in plants. Science 290: 142-144.