Required for DNA mismatch repair (MMR), correcting base-base mismatches and insertion-deletion loops (IDLs) resulting from DNA replication, DNA damage or from recombination events between non-identical sequences during meiosis. Component of the MutLalpha heterodimer that forms a ternary complex with the MutS heterodimers, which initially recognize the DNA mismatches. This complex is thought to be responsible for directing the downsteam MMR events, including strand discrimination, excision, and resynthesis. Plays a major role in maintaining the genetic stability of simple sequence repeats and in the repair of heteroduplex sites present in meiotic recombination intermediates. Does not seem to be required for homologous somatic recombination. Heterodimer of MLH1 and PMS1, called MutLalpha, which is the major MMR MutL activity correcting base-base mismatches as well as IDLs. The heterodimer binds double strand DNA independently of a mismatch with positive cooperativity and has more than one DNA binding site. Forms a ternary complex with either the MSH2-MSH6 (MutSalpha) or the MSH2-MSH3 heterodimer (MutSbeta), which recognize and bind to mismatch DNA. Ternary complex formation is promoted by ATP binding. Expressed at very low levels in mature leaves. Detected in rapidly dividing tissues. Reduced fertility. Increased homeologous recombination frequency. May be due to a competing acceptor splice site and an intron retention. Belongs to the DNA mismatch repair MutL/HexB family.