Short poly(A) tails are protected from deadenylation by the LARP1-PABP complex

Deadenylation generally constitutes the first and pivotal step in eukaryotic messenger RNA decay. Despite its importance in posttranscriptional regulations, the kinetics of deadenylation and its regulation remain largely unexplored. Here we identify La ribonucleoprotein 1, translational regulator (LARP1) as a general decelerator of deadenylation, which acts mainly in the 30-60-nucleotide (nt) poly(A) length window. We measured the steady-state and pulse-chased distribution of poly(A)-tail length, and found that deadenylation slows down in the 30-60-nt range. LARP1 associates preferentially with short tails and its depletion results in accelerated deadenylation specifically in the 30-60-nt range. Consistently, LARP1 knockdown leads to a global reduction of messenger RNA abundance. LARP1 interferes with the CCR4-NOT-mediated deadenylation in vitro by forming a ternary complex with poly(A)-binding protein (PABP) and poly(A). Together, our work reveals a dynamic nature of deadenylation kinetics and a role of LARP1 as a poly(A) length-specific barricade that creates a threshold for deadenylation.