The RNA-binding ubiquitin ligase MKRN1 functions in ribosome-associated quality control of poly(A)-translation

Human cells have evolved quality control mechanisms to ensure protein homeostasis by detecting and degrading aberrant mRNAs and protein products. A common source of aberrant mRNAs is premature polyadenylation, which can result in non-functional protein products. Translating ribosomes that encounter poly(A) sequences are terminally stalled, followed by ribosome recycling and rapid decay of the truncated nascent polypeptide via the ribosome-associated quality control (RQC). Here, we demonstrate that the conserved RNA-binding E3 ubiquitin ligase Makorin Ring Finger Protein 1 (MKRN1) promotes ribosome stalling at continuous A-tracts during RQC. Using individual-nucleotide resolution UV crosslinking and immunoprecipitation (iCLIP), we show that MKRN1 is positioned upstream of A-rich stretches and poly(A) tails in mRNAs through an interaction with the cytoplasmic poly(A)-binding protein (PABP). Ubiquitin remnant profiling uncovered PABP and ribosomal protein RPS10 as well as additional translational regulators as main substrates of MRKN1. We propose that MKRN1 serves as a first line of poly(A) recognition at the mRNA level to prevent production of erroneous proteins, thus maintaining proteome integrity. Overall design: iCLIP experiments for GFP-MKRN1 with crosslinking at 254 nm and with 4SU and crosslinking at 365 nm