Just like any other man made machine, the natural protein synthesis machinery a.k.a. the magnificent ribosome, also fails at times. Stalling midway between its duty of forming peptide bonds between various amino acids, it produces truncated/nascent proteins that are unwanted by the cell. The cell has therefore developed a mechanism to get rid of these toxic truncated proteins. Shen et. al.  found that ribosome gets partially disassembled after stalling and also looses the mRNA, followed by recruitment of Ltn1p and Rqc2p to the subunit having the nascent peptide. Ltn1p binds to the side of the ribosome where proteins are spewed out and tags them with ubiquitin, a marker for destruction. Also, Rqc2p interacts with transfer RNA binding sites and stitches only alanine and threonine to the incomplete protein, producing a CAT tail (i.e. carboxy-terminal Ala and Thr extensions). CAT tails may induce heat-shock response to ensure degradation of the truncated proteins and also protects the cell from their toxic effects. In short, when the ribosome fails, the cell still manages to sustain protein synthesis even in the absence of any genetic instructions and targets them for recycling.
- Shen PS, Park J, Qin Y, Li X, Parsawar K, Larson MH, Cox J, Cheng Y, Lambowitz AM, Weissman JS, Brandman O, Frost A. Protein synthesis. Rqc2p and 60S ribosomal subunits mediate mRNA-independent elongation of nascent chains. Science. 2015 Jan 2;347(6217):75-8.