Biology subjects

Caglayan, M.

Publications and source records attributed to Caglayan, M..

4 recordsLinked to original sources

bioRxiv2023

Structures of LIG1 active site mutants reveal the importance of DNA end rigidity for mismatch discrimination

ATP-dependent DNA ligases catalyze phosphodiester bond formation in the conserved three-step chemical reaction of nick sealing. Human DNA ligase I (LIG1) finalizes almost all DNA repair pathways following DNA polymerase-mediated nucleotide insertion. We previously reported that LIG1 discriminates mismatches depending on the architecture of the 3-terminus at a nick, however the contribution of conserved active site residues to faithful ligation remains unknown. Here, we comprehensively dissect the nick DNA substrate specificity of LIG1 active site mutants carrying Ala(A) and Leu(L) substitutions at Phe(F)635 and Phe(F)F872 residues and show completely abolished ligation of nick DNA substrates with all 12 non-canonical mismatches. LIG1EE/AA structures of F635A and F872A mutants in complex with nick DNA containing A:C and G:T mismatches demonstrate the importance of DNA end rigidity, as well as uncover a shift in a flexible loop near 5-end of the nick, which causes an increased barrier to adenylate transfer from LIG1 to the 5-end of the nick. Furthermore, LIG1EE/AA/8oxoG:A structures of both mutants demonstrated that F635 and F872 play critical roles during steps 1 or 2 of the ligation reaction depending on the position of the active site residue near the DNA ends. Overall, our study contributes towards a better understanding of the substrate discrimination mechanism of LIG1 against mutagenic repair intermediates with mismatched or damaged ends and reveals the importance of conserved ligase active site residues to maintain ligation fidelity.

biochemistry↗

bioRxiv2022

Structures of LIG1 uncover a lack of sugar discrimination against a ribonucleotide at the 3'-end of nick DNA

Human DNA ligase I (LIG1) is the main replicative ligase that seals Okazaki fragments and finalizes DNA repair pathways by joining canonical 3-OH and 5-P ends of the nick DNA in a three-step ligation reaction. Ribonucleotides can be misincorporated by DNA polymerases resulting in a nick with 3-ribonucleotide while RNase H2 mediated cleavage leaves a nick harboring 5-ribonucleotide during ribonucleotide excision repair. However, how LIG1 surveils DNA ends with a "wrong" sugar at atomic resolution is unknown. Here, we determine X-ray structures of LIG1/nick DNA complexes with 3- or 5-single ribonucleotide during different stages of the ligation reaction. Our LIG1/5-rG:C structure reveals a global conformational change, which discriminates against 5-RNA/DNA junctions at the initial step when the ligase-AMP intermediate is formed. Furthermore, we capture LIG1/3-RNA-DNA heteroduplexes that are tolerated at the active site where AMP is transferred to nick DNA (step 2) and final phosphodiester bond formation occurs (step 3). Finally, we demonstrate the mutagenic and defective ligation of the nick DNA with 3- and 5-ribonucleotide, respectively, in vitro. Together, these results uncover how LIG1 encounters ribonucleotides embedded into genome during nuclear replication and the last step of DNA repair pathways to maintain genome integrity.

biochemistry↗

bioRxiv2022

Structures of LIG1 engaging with mutagenic mismatches inserted by polβ in base excision repair

DNA ligase I (LIG1) catalyzes final ligation step following DNA polymerase (pol) {beta} gap filling and an incorrect nucleotide insertion by pol{beta} creates a nick repair intermediate with mismatched end at the downstream steps of base excision repair (BER) pathway. Yet, how LIG1 discriminates against the mutagenic 3'-mismatches at atomic resolution remains undefined. Here, we determined X-ray structures of LIG1/nick DNA complexes with G:T and A:C mismatches and uncovered the ligase strategies that favor or deter ligation of base substitution errors. Our structures revealed that LIG1 active site can accommodate G:T mismatch in a similar conformation with A:T base pairing, while it stays in the LIG1-adenylate intermediate during initial step of ligation reaction in the presence of A:C mismatch at 3'-strand. Moreover, we showed mutagenic ligation and aberrant nick sealing of the nick DNA substrates with 3'-preinserted dG:T and dA:C mismatches, respectively. Finally, we demonstrated that AP-Endonuclease 1 (APE1), as a compensatory proofreading enzyme, interacts and coordinates with LIG1 during mismatch removal and DNA ligation. Our overall findings and ligase/nick DNA structures provide the features of accurate versus mutagenic outcomes at the final BER steps where a multi-protein complex including pol{beta}, LIG1, and APE1 can maintain accurate repair.

biochemistry↗

bioRxiv2020

DNA ligase I fidelity mediates the mutagenic ligation of pol β oxidized nucleotide insertion products and base excision repair intermediates with mismatches

DNA ligase I (LIG1) completes base excision repair (BER) pathway at the last nick sealing step following DNA polymerase (pol) {beta} gap filling DNA synthesis. We previously reported that pol {beta} 8-oxo-2-deoxyribonucleoside 5-triphosphate (8-oxodGTP) insertion confounds LIG1 leading to the formation of ligation failure products with 5-adenylate (AMP) block. Here, we report the mutagenic ligation of pol {beta} 8-oxodGTP insertion products and an inefficient substrate-product channeling from pol {beta} Watson-Crick like dG:T mismatch insertion to DNA ligation by LIG1 mutant with perturbed fidelity (E346A/E592A) in vitro. Moreover, our results revealed that the substrate discrimination of LIG1 for the nicked repair intermediates with preinserted 3-8-oxodG or mismatches is governed by the mutations at both E346 and E592 residues. Finally, we found that Aprataxin (APTX) and Flap Endonuclease 1 (FEN1), as compensatory DNA-end processing enzymes, can remove 5-AMP block from the abortive ligation products with 3-8-oxodG or all possible 12 non-canonical base pairs. These findings contribute to understand the role of LIG1 as an important determinant of faithful BER, and how a multi-protein complex (LIG1, pol {beta}, APTX and FEN1) can coordinate to hinder the formation of mutagenic repair intermediates with damaged or mismatched ends at the downstream steps of the BER pathway.

biochemistry↗