**4. Conclusions**

Ricin, in particular its catalytic subunit RTA, targets one of the most important eukaryotic ribosomal catalytic centers—the GAC, which is responsible for conferring unidirectional trajectory for the translation apparatus at the expense of GTP hydrolysis driven by trGTPases. To access the GAC, ricin hijacks the ribosomal translation factor recruitment element, i.e., the P-stalk, to reach the target adenine base in the SRL on the ribosome. The stalk, composed of P-proteins, represents a unique eukaryotic element interacting with RTA, being responsible for the specificity of the toxin toward the eukaryotic ribosome. The RTA interaction with the stalk not only anchors and directs RTA towards SRL but also stimulates depurination of invariant adenine at the tip of the SRL. The SRL represents a critical ribosomal element responsible for triggering GTP hydrolysis by trGTPases. The G2659 A2660 G2661 A2662 (*E. coli* numbering) tetra-loop located on the tip of the SRL plays a key role here. Within this loop, two bases A2662 and G2661 are critical elements directly involved in stimulation of GTP hydrolysis [28]. On the other hand, A2660, i.e., a base that is depurinated by RTA, is situated away from the trGTPase active site, being a center of cooperative interaction network, contributing to stabilization of the active state of trGTPases and promoting GTP hydrolysis. Thus, the A2660, lying away from the main catalytic center, but coordinating the structural arrangemen<sup>t</sup> of the G domain of trGTPases, could hold the "power behind the throne" role. Therefore, depurination of solvent-exposed A2660 impairs the intricate interaction network and destabilizes the active state of trGTPases, which finally blocks GTP hydrolysis and, at the same time, impedes the functioning of the translational machinery. However, the majority of biochemical experiments with in vitro depurinated ribosomes have shown unequivocally that removal of A2660 blocks translation, especially at the elongation step of the translational cycle, linking the catalytic activity with ricin toxicity. Such a situation seems to be unusual inside the cell, where most of the toxin molecules are degraded during the ricin tra fficking and, as it was estimated, only up to 5% of RTA molecules reach the endoplasmic reticulum [132–134]. Thus, ricin must have evolved to be extremely successful in winning the battle to hurt the GAC - the energetic heart of the ribosome; this is especially well demonstrated by the higher a ffinity towards the ribosomal P-stalk proteins [21,86,102] compared to translational factors [82]. Importantly, in vivo analyses did not find any clear link between the ribosome depurination, translation inhibition, and cell death, indicating that molecular events contributing to the so-called 'cause and e ffect' of the ricin *modus operandi* are still obscure.

**Author Contributions:** P.G. and M.T. conceived the review; P.G. and M.S. performed the literature review; P.G. and M.S. prepared the figures; P.G., M.S., P.H.-D. and M.T. wrote the manuscript.

**Funding:** This work was supported by the National Science Centre in Poland, gran<sup>t</sup> number UMO-2014/13/B/ NZ1/00953 to M.T. and UMO-2017/01/X/NZ1/01674 to P.G.

**Conflicts of Interest:** The authors declare no conflict of interest.
