Exonuclease 1

Exonuclease 1 is an enzyme that in humans is encoded by the EXO1 gene.^[5][6][7]

EXO1
Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 3QE9, 3QEA, 3QEB
Identifiers
Aliases	EXO1, HEX1, hExoI, exonuclease 1
External IDs	OMIM: 606063; MGI: 1349427; HomoloGene: 31352; GeneCards: EXO1; OMA:EXO1 - orthologs
Gene location (Human) Chr. Chromosome 1 (human)[1] Band 1q43 Start 241,847,967 bp[1] End 241,895,148 bp[1]
Gene location (Mouse) Chr. Chromosome 1 (mouse)[2] Band 1 H4|1 81.9 cM Start 175,708,147 bp[2] End 175,741,055 bp[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in ventricular zone gonad ganglionic eminence testicle trabecular bone bone marrow bone marrow cells stromal cell of endometrium secondary oocyte mucosa of transverse colon Top expressed in epiblast primary oocyte zygote embryo nasolacrimal duct morula secondary oocyte embryo spermatocyte tail of embryo More reference expression data BioGPS More reference expression data
Gene ontology Molecular function endonuclease activity double-stranded DNA 5'-3' exodeoxyribonuclease activity single-stranded DNA 5'-3' exodeoxyribonuclease activity DNA binding 5'-3' exonuclease activity nuclease activity RNA-DNA hybrid ribonuclease activity protein binding 5'-3' exodeoxyribonuclease activity flap endonuclease activity chromatin binding exonuclease activity hydrolase activity, acting on ester bonds catalytic activity hydrolase activity metal ion binding double-stranded DNA binding 5'-flap endonuclease activity Cellular component nucleus nucleoplasm nuclear body plasma membrane Biological process DNA recombination immune system process humoral immune response mediated by circulating immunoglobulin DNA mismatch repair cellular response to DNA damage stimulus somatic hypermutation of immunoglobulin genes meiosis isotype switching DNA repair RNA phosphodiester bond hydrolysis, endonucleolytic nucleic acid phosphodiester bond hydrolysis DNA replication t-circle formation meiotic DNA double-strand break processing regulation of signal transduction by p53 class mediator Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 9156 26909 Ensembl ENSG00000174371 ENSMUSG00000039748 UniProt Q9UQ84 Q5T397 Q9QZ11 RefSeq (mRNA) NM_003686 NM_006027 NM_130398 NM_001319224 NM_012012 RefSeq (protein) NP_001306153 NP_003677 NP_006018 NP_569082 NP_036142 Location (UCSC) Chr 1: 241.85 – 241.9 Mb Chr 1: 175.71 – 175.74 Mb PubMed search [3] [4]
Wikidata
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This gene encodes a protein with 5' to 3' exonuclease activity as well as RNase activity (endonuclease activity cleaving RNA on DNA/RNA hybrid).^[8] It is similar to the Saccharomyces cerevisiae protein Exo1 which interacts with Msh2 and which is involved in DNA mismatch repair and homologous recombination. Alternative splicing of this gene results in three transcript variants encoding two different isoforms.^[7]

Meiosis

A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.

ExoI is essential for meiotic progression through metaphase I in the budding yeast Saccharomyces cerevisiae and in mouse.^[9][10]

Recombination during meiosis is often initiated by a DNA double-strand break (DSB) as illustrated in the accompanying diagram. During recombination, sections of DNA at the 5' ends of the break are cut away in a process called resection. In the strand invasion step that follows, an overhanging 3' end of the broken DNA molecule "invades" the DNA of a homologous chromosome that is not broken, forming a displacement loop (D-loop). After strand invasion, the further sequence of events may follow either of two main pathways leading to a crossover (CO) or a non-crossover (NCO) recombinant (see Genetic recombination and Homologous recombination). The pathway leading to a CO involves a double Holliday junction (DHJ) intermediate. Holliday junctions need to be resolved for CO recombination to be completed.

During meiosis in S. cerevisiae, transcription of the Exo1 gene is highly induced.^[9] In meiotic cells, Exo1 mutation reduces the processing of DSBs and the frequency of COs.^[9] Exo1 has two temporally and biochemically distinct functions in meiotic recombination.^[11] First, Exo1 acts as a 5’–3’ nuclease to resect DSB-ends. Later in the recombination process, Exo1 acts to facilitate the resolution of DHJs into COs, independently of its nuclease activities. In resolving DHJs, Exo 1 acts together with MLH1-MLH3 heterodimer (MutL gamma) and Sgs1 (ortholog of Bloom syndrome helicase) to define a joint molecule resolution pathway that produces the majority of crossovers.^[12]

Male mice deficient for Exo1 are capable of normal progress through the pachynema stage of meiosis, but most germ cells fail to progress normally to metaphase I due to dynamic loss of chiasmata.^[10] Surprisingly though, this meiotic role of Exo1 is not mediated by its nuclease activity per se, since Exo1-DA mice harboring a point mutation in Exo1's nuclease domain have no detectable meiotic defects.^[13]

Interactions

Exonuclease 1 has been shown to interact with MSH2^[6][14][15] and MLH1.^[15]