Figure 4.

Degradation pathway for the non-functional ribosome. The decoding-defective ribosomes are initially monoubiquitinated by Mag2 and then polyubiquitinated by Fap1 to initiate 18S nonfunctional rRNA decay (selective degradation of small subunit). In the first selection, Mag2 recognizes the slow translating ribosome and monoubiquitinates ribosomal protein uS3. After the first selection step, Fap1 recognizes stalling monoribosomes and elongates the K63-linked polyubiquitin chain on the Mag2-mediated monoubiquitinated uS3. This tandem ubiquitin system triggers the non-functional 18S rRNA decay.