Structural Insight into BLM Recognition by TopBP1

Abstract

SUMMARY Topoisomerase IIβ binding protein 1 (TopBP1) is a critical protein-protein interaction hub in DNA replication checkpoint control. It was proposed that TopBP1 BRCT5 interacts with Bloom syndrome helicase (BLM) to regulate genome stability through either phospho-Ser304 or phospho-Ser338 of BLM. Here we show that TopBP1 BRCT5 specifically interacts with the BLM region surrounding pSer304, not pSer338. Our crystal structure of TopBP1 BRCT4/5 bound to BLM reveals recognition of pSer304 by a conserved pSer-binding pocket, and interactions between a FVPP motif N-terminal to pSer304 and a hydrophobic groove on BRCT5. This interaction utilizes the same surface of BRCT5 that recognizes the DNA damage mediator, MDC1, however the binding orientations of MDC1 and BLM are reversed. While the MDC1 interactions are largely electrostatic, the interaction with BLM has higher affinity and relies on a mix of electrostatics and hydrophobicity. We suggest similar evolutionarily conserved interactions may govern interactions between TopBP1 and 53BP1.

Treacle controls the nucleolar response to rDNA breaks via TOPBP1 recruitment and ATR activation

“…In addition, amino acids N-terminal of this SQ motif closely resemble the TOPBP1 BRCT4-5 interaction site in BLM ( Supplementary Fig. 9d) 37,38 .…”

Phosphoproteomics Reveals a Distinct Mode of Mec1/ATR Signaling in Response to DNA End Hyper-Resection

“…Interestingly, experiments using the B3/4-BLM Human chimera suggest a conserved role for the STR-Dpb11 interaction in regulating recombination. Indeed, BLM and TOPBP1 were reported to interact in human cells (Blackford et al, 2015; Sun et al, 2017), although it is unclear whether ATR plays any role in promoting that interaction, and if the BLM-TOPBP1 interaction affects HR.…”

A New BRCT Binding Mode in TopBP1-BLM Helicase Interaction

“…Interestingly, recent work suggested that a single BRCT domain (BRCT5) mediates the TopBP1-BLM interaction and that two different phosphorylated serine sites in BLM, pSer304 (Blackford et al, 2015) and pSer338 (Wang et al, 2013), could be the targets of BRCT5. In this issue of Structure , Sun et al (2017) show that TopBP1 BRCT5 recognizes BLM pSer304 but not pSer338. Consistent with this finding, two acidic and four hydrophobic residues preceding Ser304 are evolutionarily conserved, while residues surrounding Ser338 are not conserved.…”

“…Overall, with the demonstration that a single BRCT domain (TopBP1 BRCT5) can bind a phosphorylated target (BLM helicase) with a micromolar range K d , on par with the typical affinities measured for tandem BRCT repeats, the study of Sun et al (2017) deepens our understanding of phosphotarget recognition by BRCT domains. This work also strongly suggests that a similar mechanism is at play in the interaction of TopBP1 with 53BP1, a key DNA double-strand break repair protein whose association with TopBP1 may contribute to its DNA damage checkpoint function (Qu et al, 2013).…”

“…In this issue of Structure , Sun et al (2017) show that a single BRCT domain in TopBP1 binds tightly and specifically to phosphorylated Bloom syndrome helicase (BLM). This work reveals a novel BRCT binding mode and suggests a similar mechanism for TopBP1 interaction with 53BP1.…”