Human Ape2 protein has a 3'-5' exonuclease activity that acts preferentially on mismatched base pairs

Abstract

DNA damage, such as abasic sites and DNA strand breaks with 3′-phosphate and 3′-phosphoglycolate termini present cytotoxic and mutagenic threats to the cell. Class II AP endonucleases play a major role in the repair of abasic sites as well as of 3′-modified termini. Human cells contain two class II AP endonucleases, the Ape1 and Ape2 proteins. Ape1 possesses a strong AP-endonuclease activity and weak 3′-phosphodiesterase and 3′–5′ exonuclease activities, and it is considered to be the major AP endonuclease in human cells. Much less is known about Ape2, but its importance is emphasized by the growth retardation and dyshematopoiesis accompanied by G2/M arrest phenotype of the APE2-null mice. Here, we describe the biochemical characteristics of human Ape2. We find that Ape2 exhibits strong 3′–5′ exonuclease and 3′-phosphodiesterase activities and has only a very weak AP-endonuclease activity. Mutation of the active-site residue Asp 277 to Ala in Ape2 inactivates all these activities. We also demonstrate that Ape2 preferentially acts at mismatched deoxyribonucleotides at the recessed 3′-termini of a partial DNA duplex. Based on these results we suggest a novel role for human Ape2 as a 3′–5′ exonuclease.

Differential expression of APE1 and APE2 in germinal centers promotes error-prone repair and A:T mutations during somatic hypermutation

“…Second, it is unlikely that AP sites would go unrepaired and encounter DNA polymerase given normal levels of APE1 expression. The enzymatic activity of APE1 for AP-site repair has been reported to be from 70-to 450,000-fold more efficient than that of APE2 (20,21,42). In addition, APE1 interacts directly with XRCC1 (30,43), a scaffolding protein that recruits Pol β and ligase to promote SSB repair.…”

“…APE1 is the major APE; it is ubiquitously expressed and essential for early embryonic development in mice and for viability of human cell lines (17)(18)(19). APE1 has strong endonuclease activity and weaker 3′-5′ exonuclease (proofreading) and 3′-phosphodiesterase (end-cleaning) activities (20,21). Recombinant purified human APE2 has much weaker AP endonuclease activity than APE1, but its 3′-5′ exonuclease activity is strong compared with APE1, although it is not processive (20).…”

“…APE1 has strong endonuclease activity and weaker 3′-5′ exonuclease (proofreading) and 3′-phosphodiesterase (end-cleaning) activities (20,21). Recombinant purified human APE2 has much weaker AP endonuclease activity than APE1, but its 3′-5′ exonuclease activity is strong compared with APE1, although it is not processive (20). However, APE2 has been shown to interact with proliferating Significance Error-prone DNA repair of activation-induced cytidine deaminase (AID)-induced lesions in B cells is known to cause hypermutation of the antibody genes and, when coupled with selection mechanisms in germinal centers, leads to increased affinity of antibody.…”

Human Apurinic/Apyrimidinic Endonuclease 1

“…Instead, there is a second protein with homology to exonuclease III, termed APE2 (also known as APEX2) (71,208). The AP endonuclease and 3¢-damage excision activities of the human APE2 protein are generally weak, and, thus, its function as a DNA repair enzyme remains in question (18,19,70). It is not our intent to describe the current picture of APE2 in detail, but we point out that, while APE2-null mice exhibit growth retardation and dyshematopoiesis, APE2-deficient mouse embryonic stem cells or immortalized fibroblasts do not display increased sensitivity to several genotoxic agents, including hydrogen peroxide, bleomycin, or x-ray irradiation (86).…”

The role of DNA exonucleases in protecting genome stability and their impact on ageing

“…APE1 removes nucleotides from matched, mismatched and beta-l-dioxolane-cytidine nicked DNA Cheng 2002, 2003), with a marked preference for 3′-mismatched and nicked substrates. APE2 also has a preference for removal of mismatched bases (Burkovics et al 2006). It remains to be determined whether and how much these proteins contribute to proofreading during replication or whether they utilise this functionality only within the context of DNA repair.…”