ABCE1: A special factor that orchestrates translation at the crossroad between recycling and initiation

Abstract

For many years initiation and termination of mRNA translation have been studied separately. However, a direct link between these 2 isolated stages has been suggested by the fact that some initiation factors also control termination and can even promote ribosome recycling; i.e. the last stage where post-terminating 80S ribosomes are split to start a new round of initiation. Notably, it is now firmly established that, among other factors, ribosomal recycling critically requires the NTPase ABCE1. However, several earlier reports have proposed that ABCE1 also somehow participates in the initiation complex assembly. Based on an extended analysis of our recently published late-stage 48S initiation complex from rabbit, here we provide new mechanistic insights into this putative role of ABCE1 in initiation. This point of view represents the first structural evidence in which the regulatory role of the recycling factor ABCE1 in initiation is discussed and establishes a corner stone for elucidating the interplay between ABCE1 and several initiation factors during the transit from ribosomal recycling to formation of the elongation competent 80S initiation complex.

Keywords: ABCE1; cryo-EM; recycling; ribosome; translation.

Figure 1.

Fitting of ABCE1 in our late-stage m48S initiation complex. (A) Atomic model of the m48S late-stage IC fitted in its cryo-EM segmented map (middle panel), focused on the densities attributed to ABCE1, seen from the intersubunit face. Right panel, close-up on the atomic model ABCE1 fitted in its density map. Left panel, Blow-up on the FeS clusters domain of ABCE1 at high contour-threshold of the map showing the coincidence of the latter clusters with the highest density spots, which supports our model. (B) Schematic model of the observed conformational rearrangement of ABCE1 in the m48S late-stage complex compared with the P. abyssi crystal structure. (C) Partial overlapping between eIF3b subunit and the FeS domain of ABCE1, seen from the intersubunit face, highlighted by dashed cicle.

Figure 2.

Model of the ABCE1 activity during translation initiation. We propose that the ABCE1, depicted in green, binds to either the 40S–deacylated tRNA–mRNA recycling intermediate along with eIFs 1, 1A and 3, which will then capture the ribosomal recycling stage by producing the free post-recycled 40S complex, or past this step directly to the post-recycled 40S-eIF1-eIF1A-eIF3 complex. During the pre-initiation and initiation stages ABCE1 may act as the critical anti-association factor by cycling on and off the ribosome in an NTP-dependent manner, unless its binding site at the GTPase binding region of the 40S subunit is occupied by other factors such as the eIF3b subunit. At a late initiation stage, eIF5B prevents ABCE1 from further cycling to promote proper subunit joining followed by GTP hydrolysis and its own release. Thick curved gray arrows indicate the eIF3b-g-i module relocation from the solvent to the intersubunit side of the 40S subunit. Red arrow indicates the conformational change of the eIF2-TC upon eIF3b relocation and departure to and from the intersubunit face of the 40S, which may reflect its described previously P^IN and P^OUT conformations. A question mark indicates an unknown fate of eIF3 that has been proposed to remain bound to the 40S subunit even past subunit joining PMID:18765792; PMID:18056426).