TOPBP1

It accomplishes these interactions with other protein partners through its breast cancer associated gene 1 C-terminus (BRCT) domains.

[10] A BRCT domain is structurally defined by a 4 member β sheet that is bookended by one α-helix (α2) and two other α-helices (α1 and α3).

The amino acid residues that make up these core features are highly conserved, with protein specific deviations occurring in the loops that connect these subunits.

When DNA damage was induced at higher levels by γ irradiation, there was an increase in TOPBP1/BRCA1 at sites away from replication forks.

The junction of RPA coated ssDNA and intact double stranded DNA (dsDNA) is where TOPBP1 and the 9-1-1 clamp is recruited.

[17][18][19] In addition to TOPBP1, ATR has also been found to be activated by the ssDNA specific, RPA interacting protein ETAA1.

[21]TOPBP1/9-1-1 recruitment is conducted independent of ATRIP/ATR which serves as a regulatory mechanism that prevents both premature and non-specific activation of the DNA damage response pathway.

Knockdowns of TOPBP1 gene expression leads to a reduction in phosphorylation of downstream ATR kinase targets.

[17][19] Following ATR activation, it is able to phosphorylate downstream DNA damage associated factors, with the primary effector being the kinase Chk1.

In summary, TOPBP1 acts as a scaffolding protein that facilitates the interactions necessary to form the DNA replication pre-initiation complex.

The E2F family of transcription factors mediate the expression of a multitude of genes involved in a variety of functions.

[17] The induction of a repressive transcriptional state in apoptotic related genes is thought to be from the TOPBP1 mediated recruitment of chromatin remodeling machines, e.g. histone deacetylases (HDAC).

This phenomenon may be caused by oncogenic induced activation, difficult to replicate structures, transcription/replication collisions, polymerase uncoupling, dNTP starvation, and other sources.

TOPBP1 is responsible for recruiting the MiDAS essential scaffolding protein SLX4, which forms a large nuclease complex.

This suggests that UFBs are a normal outcome of mitosis and that TOP2A may play a role in resolving them before the cell exits the cell cycle thereby preventing adverse outcomes[8] TOPBP1 was found to localize to both UFBs and co-localize with TOP2A, which is a conserved interaction found in the yeast homologue Dpb11.

In contrast to this finding, another study found a decrease in the gene expression of TOPBP1 by RT-PCR in 127 breast cancer patients.

In addition, this study found that TOPBP1 was aberrantly expressed in the cytoplasm in this cohort of familial breast cancer patients.

[10][17] TOPBP1 overexpression is associated with advanced stage sarcomas, lung metastasis, and chemoresistance to platinum agents (e.g.

[10] Utilizing publicly available datasets of whole-exome sequencing, a link was found between TOPBP1 mutations and pulmonary hypertension (PAH).

[28] In follow up studies, knockdown of TOPBP1 by siRNA led to an increase in detectable DNA damage and apoptosis in healthy pulmonary endothelial cells.

Structure of TOPBP1 organized by its BRCT domains. BRCT domains 0 + 1 + 2 (6HM5), BRCT domains 4 + 5 ( 3UEO), BRCT domain 6 (3JVE), BRCT domains 7 + 8 (3AL2). [ 12 ]
TOPBP1 Activation of ATR in DNA Damage Response
As the cell enters S phase, the MCM complex is loaded onto double stranded DNA in anticipation of replication. Treslin is phosphorylated by the cell cycle checkpoint kinase CDK2, which allows it to interact with TOPBP1. The Treslin-TOPBP1 complex recruits CDC45 to the MCM complex and DNA replication is officially initiated.
Following DNA damage, TOPBP1 is phosphorylated by Akt and self oligomerizes at the BRCT7/8 domains. Oligomeric TOPBP1 can bind to E2F-1 and recruit chromatin remodeling machinery (e.g. HDAC) to repress E2F-1 driven gene expression.