*Review* **How Ricin Damages the Ribosome**

#### **Przemysław Grela \*, Monika Szajwaj, Patrycja Horbowicz-Dro ˙zd ˙zal and Marek Tchórzewski**

Department of Molecular Biology, Maria Curie-Skłodowska University, Akademicka 19, 20-033 Lublin, Poland; monika.szajwaj@poczta.umcs.lublin.pl (M.S.); patrycja.horbowicz@gmail.com (P.H.-D.); maro@hektor.umcs.lublin.pl (M.T.)

**\*** Correspondence: przemek@hektor.umcs.lublin.pl; Tel.: +48-81-537-5954

Received: 31 March 2019; Accepted: 24 April 2019; Published: 27 April 2019

**Abstract:** Ricin belongs to the group of ribosome-inactivating proteins (RIPs), i.e., toxins that have evolved to provide particular species with an advantage over other competitors in nature. Ricin possesses RNA N-glycosidase activity enabling the toxin to eliminate a single adenine base from the sarcin-ricin RNA loop (SRL), which is a highly conserved structure present on the large ribosomal subunit in all species from the three domains of life. The SRL belongs to the GTPase associated center (GAC), i.e., a ribosomal element involved in conferring unidirectional trajectory for the translational apparatus at the expense of GTP hydrolysis by translational GTPases (trGTPases). The SRL represents a critical element in the GAC, being the main triggering factor of GTP hydrolysis by trGTPases. Enzymatic removal of a single adenine base at the tip of SRL by ricin blocks GTP hydrolysis and, at the same time, impedes functioning of the translational machinery. Here, we discuss the consequences of SRL depurination by ricin for ribosomal performance, with emphasis on the mechanistic model overview of the SRL *modus operandi*.

**Keywords:** ribosome-inactivating protein; ricin; ribosome; sarcin-ricin loop (SRL); GTPase associated center (GAC); P-proteins; translation

**Key Contribution:** This review summarizes the current knowledge of ricin-induced impairment of the ribosome and translation process and highlights important aspects in this field.
