DNA Ligase III (LIG3) is a specialized DNA ligase that plays critical roles in DNA repair pathways essential for maintaining genomic integrity in both the nucleus and mitochondria. The enzyme catalyzes the final step of DNA strand ligation in base excision repair (BER), single-strand break repair (SSBR), and mitochondrial DNA repair[1]. In neurons, where DNA damage accumulates throughout life due to high metabolic activity and oxidative stress, LIG3 function is essential for neuronal survival and has been implicated in Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis[2].
| DNA Ligase III | |
|---|---|
| Protein Name | DNA Ligase III, LIG3 |
| Gene | [LIG3](/genes/lig3) |
| UniProt ID | [P18858](https://www.uniprot.org/uniprot/P18858) |
| PDB Structures | 3KUE, 3V2N |
| Molecular Weight | ~102 kDa (alpha), ~68 kDa (mitochondrial) |
| Protein Length | 755 amino acids (full-length) |
| Subcellular Localization | Nucleus, Mitochondria |
| Protein Family | DNA ligase family |
| Chromosomal Location | 17q12 |
DNA ligases are essential enzymes that catalyze the formation of phosphodiester bonds between adjacent 3'-hydroxyl and 5'-phosphate ends in DNA molecules. LIG3 is unique among eukaryotic DNA ligases in that it exists in multiple isoforms with distinct subcellular localizations: a nuclear isoform that functions in association with the XRCC1 scaffold protein, and a mitochondrial isoform that lacks XRCC1 interaction and is essential for mitochondrial DNA (mtDNA) maintenance[3].
The importance of LIG3 in the nervous system cannot be overstated. Neurons are post-mitotic cells that must maintain genomic integrity throughout an organism's lifetime without the benefit of cell division to replace damaged cells. The brain is particularly vulnerable to oxidative DNA damage due to its high metabolic rate, high oxygen consumption, and abundant lipid content. LIG3-mediated DNA repair is the primary pathway for repairing the oxidative DNA damage that accumulates continuously in neurons[@neuron dna repair].
LIG3 contains multiple functional domains that enable its DNA repair functions[4]:
| Domain | Position | Function |
|---|---|---|
| Zinc finger (ZFD) | 1-70 aa | DNA binding, DNA damage recognition |
| BRCT domain | 220-300 aa | Protein-protein interactions, XRCC1 binding |
| Catalytic core | 301-600 aa | Adenylation, phosphodiester bond formation |
| C-terminal region | 601-755 aa | Isoform-specific targeting, regulation |
LIG3 catalyzes DNA ligation through a three-step mechanism:
Key structural features include:
| Isoform | Length | Localization | XRCC1 Interaction | Function |
|---|---|---|---|---|
| LIG3α | 755 aa | Nucleus | Yes | Nuclear BER |
| LIG3β | ~68 kDa | Mitochondria | No | mtDNA repair |
| LIG3γ | Testis-specific | Testis | Unknown | Meiosis |
LIG3 participates in several DNA repair pathways[@ber pathway]:
BER is the primary pathway for repairing small, non-helix-distorting DNA lesions, including:
The BER pathway proceeds as follows:
LIG3 is also essential for repairing single-strand breaks that arise from:
SSBR shares many proteins with BER and proceeds through similar steps.
LIG3 functions in close association with the XRCC1 scaffold protein[@xrcc1 ligase]:
| XRCC1 Domain | LIG3 Interaction | Function |
|---|---|---|
| N-terminal | LIG3 BRCT domain | Protein recruitment |
| Central region | PARP, DNA polymerase β | Platform assembly |
| C-terminal | LIG3 BRCT domain | Final ligation |
The XRCC1-LIG3 complex ensures efficient repair by:
The mitochondrial isoform of LIG3 (LIG3β) is essential for mtDNA maintenance[mitochondrial ligase]:
Unlike nuclear LIG3, mitochondrial LIG3 does not interact with XRCC1 and is targeted to mitochondria through an alternative translation initiation site.
Neurons have specialized DNA repair mechanisms[@neuron dna repair]:
LIG3 dysfunction is strongly implicated in AD pathogenesis[@ad dna repair]:
In PD, LIG3 plays important roles in dopaminergic neuron survival[pd dna repair]:
Dopaminergic neurons are particularly vulnerable due to:
LIG3 dysfunction contributes to ALS pathogenesis[als dna repair]:
While not a neurodegenerative disease, LIG3 has complex roles in cancer[lig3 therapeutics]:
| Partner | Interaction | Functional Significance |
|---|---|---|
| XRCC1 | BRCT domain | Scaffold for nuclear BER |
| PARP1/2 | Direct binding | DNA damage sensing |
| DNA polymerase β | Sequential | Gap filling |
| AP endonuclease | Sequential | Nick processing |
| PCNA | Replication | Cell cycle coordination |
| Approach | Mechanism | Status | Indication |
|---|---|---|---|
| LIG3 activators | Enhance ligase activity | Discovery | AD, PD |
| Gene therapy | Restore LIG3 expression | Research | ALS |
| PARP inhibitors | Synthetic lethality | Approved | Cancer |
| Antioxidants | Reduce DNA damage | Clinical | Neurodegeneration |
| Region | Expression Level | Significance |
|---|---|---|
| Hippocampus | High | Learning/memory, vulnerable in AD |
| Cerebral cortex | High | Cognitive function |
| Substantia nigra | High | Dopaminergic neurons, vulnerable in PD |
| Spinal cord | High | Motor neurons, vulnerable in ALS |
| Cerebellum | Moderate | Motor coordination |
| Damage Type | Repair Pathway | Neurodegenerative Relevance |
|---|---|---|
| 8-oxoguanine | BER | AD, PD |
| Single-strand breaks | BER/SSBR | AD, PD, ALS |
| Double-strand breaks | NHEJ | ALS |
| Mitochondrial mutations | mtDNA repair | PD |
| Bulky lesions | NER | Rare in neurons |
Neurons exhibit unique DNA damage response characteristics:
Ellenberger T, et al. Eukaryotic DNA ligases: structural and functional insights. Annual Review of Biochemistry. 2008. ↩︎
Shrivastava AN, et al. DNA damage and repair in neurodegenerative diseases. Progress in Neurobiology. 2018. ↩︎
Lakshmipathy U, Campbell C. The human DNA ligase III gene encodes nuclear and mitochondrial proteins. Molecular and Cellular Biology. 1999. ↩︎
Georgiadis MM, et al. Structure of human DNA ligase III. Acta Crystallographica D. 2008. ↩︎