Structure of CRL2Lrr1, the E3 ubiquitin ligase that promotes DNA replication termination in vertebrates

When vertebrate replisomes from neighboring origins converge, the Mcm7 subunit of the replicative helicase, CMG, is ubiquitylated by the E3 ubiquitin ligase, CRL2(Lrr1). Polyubiquitylated CMG is then disassembled by the p97 ATPase, leading to replication termination. To avoid premature replisome disassembly, CRL2(Lrr1) is only recruited to CMGs after they converge, but the underlying mechanism is unclear. Here, we use cryogenic electron microscopy to determine structures of recombinant Xenopus laevis CRL2(Lrr1) with and without neddylation. The structures reveal that CRL2(Lrr1) adopts an unusually open architecture, in which the putative substraterecognition subunit, Lrr1, is located far from the catalytic module that catalyzes ubiquitin transfer. We further demonstrate that a predicted, flexible pleckstrin homology domain at the N-terminus of Lrr1 is essential to target CRL2(Lrr1) to terminated CMGs. We propose a hypothetical model that explains how CRL2(Lrr1) 's catalytic module is positioned next to the ubiquitylation site on Mcm7, and why CRL2(Lrr1) binds CMG only after replisomes converge.