Fig. 6: A model of Polθ-hel roles at the initial steps in MMEJ.
From: Structural basis for Polθ-helicase DNA binding and microhomology-mediated end-joining
The two ends of double-stranded DNA breaks (DSBs) are shown at the top, with RPA binding to the resected 3′-ssDNA overhangs. Polθ forms a dimer via Polθ-hel (each protomer labeled as “H” and colored in green and cyan), with Polθ-pol (labeled as “P”) tethered to the helicase dimer through the flexible Polθ-ctr (labeled as “C”). Step 1: Each protomer of the Polθ-hel dimer binds to one of the two resected 3′-ssDNA overhangs, bringing the opposing 3′-ssDNA ends close to each other at the dimer cleft, allowing for microhomology search and annealing. Polθ-hel ATPase activity can also displace RPA from 3′-ssDNA overhangs. Step 2: Polθ-hel samples the ssDNA sequence microhomology by translocating along the ssDNA and anneals the ssDNA ends at a location with sufficient microhomology. Steps 3, 4: One Polθ-pol protomer could then bind to the short annealed microhomology dsDNA, and extend the primer. Step 5, 6: When the first Polθ-pol protomer extends the primer to a certain length, the second Polθ-pol can bind and extend the opposing primer in the opposite direction to complete the polymerization step. Step 7: In addition to Polθ, Polδ, PCNA, DNA ligase, and other protein factors promote final processing and sealing of the DSB to complete MMEJ repair.
