Structure of human CST-pol-α/primase bound to a telomeric overhang poised for initiation of telomere C-strand synthesis

This is the authors' abstract. We don't add key points for this paper.

Telomere replication and regulation protect mammalian chromosome ends and promote genome stability. An essential step in telomere maintenance is the C-strand fill-in process, which is the de novo synthesis of the complementary strand of the telomere overhang. This step is catalyzed by polymerase-alpha/primase complex (pol-α/primase) and coordinated by an accessory factor, CTC1-STN1-TEN1 (CST). Using cryogenic-electron microscopy single-particle analysis, we report the structure of the human telomere C-strand fill-in preinitiation complex (PIC) at 3.9 Å resolution. The structure reveals a CST and a pol-α/primase co-bound to a single telomere overhang, poised for de novo RNA primer synthesis. Upon PIC assembly, the pol-α/primase undergoes large conformation change from its apo-state; CST partitions the DNA and RNA catalytic centers of pol-α/primase into two separate domains and positions the 3 ′ end of an extended telomere single-stranded DNA template towards the RNA catalytic center (PRIM1 or p49). The telomeric single-stranded DNA template is further positioned by the POLA1 (or p180) catalytically dead exonuclease domain. Together with CST, the exonuclease domain forms a tight-fit molecular tunnel for template direction. Given the structural homology of CST to Replication Protein A (RPA), our structure provides the structural basis for a new model of how pol-α/primase lagging-strand DNA synthesis is coordinated by single-stranded DNA-binding accessory factors.