TOPBP1

TOPBP1
Name	TOPBP1
Organism	Homo sapiens

TOPBP1 TOPBP1 is a human nuclear protein involved in DNA replication and the DNA damage response. It functions as a scaffold that coordinates interactions among multiple repair, checkpoint, and replication factors to maintain genomic stability. TOPBP1 activity is critical during S phase and mitosis and is implicated in cancer biology and developmental disorders.

Structure and domains

TOPBP1 contains multiple BRCT (BRCA1 C Terminus) motifs that form tandem domain arrays similar to those in BRCA1, BRCA2, 53BP1, MDC1, and PALB2. The protein architecture includes an N‑terminal region, central BRCT repeats, and a C‑terminal ATR activation domain that resembles activation modules found in RAD9, RAD17, HUS1 and other checkpoint proteins. Crystal and cryo‑EM studies comparing BRCT folds from XPA, XRCC1, FANCD2 and BRIP1 reveal conserved phosphopeptide‑binding pockets that mediate phospho‑dependent interactions. Post‑translational modification sites overlap with motifs recognized by kinases such as ATM, ATR, CHK1, and CDC7.

Function and mechanisms

TOPBP1 acts as a multivalent scaffold to recruit enzymes and adaptors during replication initiation and checkpoint signaling. It activates the ATR kinase by directly engaging ATRIP‑ATR complexes similarly to mechanisms described for Topoisomerase II regulators and for ATR activators in yeast such as Dpb11 and Sld2. Its BRCT domains recognize phosphorylated motifs generated by CDK1, CDK2, PLK1 and CHK2, enabling ordered assembly of complexes at stalled forks and DNA lesions. TOPBP1 regulates origin firing in coordination with origin recognition factors including ORC1, CDC6, MCM2-7 and the GINS complex, coupling cell‑cycle cues from Cyclin A and Cyclin E to replication initiation.

Role in DNA damage response and replication

TOPBP1 is essential for S‑phase checkpoint enforcement and replication stress tolerance, functioning alongside ATR, ATRIP, RAD17, Claspin, and RPA. At sites of DNA damage TOPBP1 is recruited with sensor proteins such as MRE11, RAD50, NBS1, DNA‑PKcs and Ku70 to activate signaling cascades that phosphorylate effectors including CHK1 and p53. During homologous recombination TOPBP1 coordinates components like BRCA1, BRCA2, RAD51, and PALB2 to support strand invasion and fork restart. It also participates in mitotic DNA repair pathways that involve Aurora B, PLK1, and BubR1 to preserve chromosome segregation fidelity.

Interactions and protein partners

TOPBP1 interacts with a wide network of proteins: checkpoint mediators RAD9, HUS1, and RAD1 (9‑1‑1 complex); recombination factors RAD51, BRCA1, and BRCA2; replication initiators CDC45, MCMs, and GINS; and signaling kinases ATR, ATM, CHK1, and CHK2. It also binds chromatin regulators such as Histone H2AX, MDC1, RNF8, and RNF168 during ubiquitin‑mediated signaling, and engages transcription factors like E2F1 and chromatin remodelers including SMARCA4 (BRG1) for replication–transcription coordination. Ubiquitin and SUMO pathway enzymes such as UBC13 and SENP1 modulate TOPBP1 interactions.

Clinical significance and disease associations

Alterations in TOPBP1 expression or mutation of interaction interfaces are associated with genomic instability observed in breast cancer, ovarian cancer, colorectal cancer, and leukemia. TOPBP1 dysregulation impacts sensitivity to DNA‑damaging chemotherapies and targeted inhibitors of ATR, PARP1, and CHK1, influencing treatment responses in clinical trials led by institutions like NCI and Memorial Sloan Kettering Cancer Center. Germline and somatic variants affecting BRCT domains or the ATR activation region have been reported in cohorts studied by consortia such as TCGA and ICGC, and are implicated in developmental disorders analogous to defects seen in Seckel syndrome and Fanconi anemia pathways. TOPBP1 is therefore a candidate biomarker for prognosis and a potential therapeutic target in precision oncology programs at centers including MD Anderson Cancer Center.

Evolution and homologs

TOPBP1 is conserved across eukaryotes; orthologs include Dpb11 from Saccharomyces cerevisiae, Cut5 from Schizosaccharomyces pombe, and homologs in metazoans such as Drosophila melanogaster and Caenorhabditis elegans. Comparative genomics studies involving resources like Ensembl, NCBI, and UniProt show conservation of BRCT domains and the ATR activation motif, with lineage‑specific expansions in plants and protists. Evolutionary analyses referencing datasets from 1000 Genomes Project and Phytozome highlight domain rearrangements that correlate with organismal complexity and replication program diversity.

Category:DNA repair proteins