Human MutLα: The jack of all trades in MMR is also an endonuclease

Abstract

SummaryRecently, Paul Modrich's group reported the discovery of an intrinsic endonuclease activity for human MutLα. This breakthrough provides a satisfactory answer to the longstanding puzzle of a missing nuclease activity in human mismatch repair and will undoubtedly lead to new investigations of DNA repair and replication. Here, the implications of this exciting new finding are discussed in the context of mismatch repair in E. coli and humans.Correction of replication errors by mismatch repair (MMR) has long been recognized as critical for genomic stability. Inactivation of MMR in humans has been implicated in > 90% of hereditary nonpolyposis colorectal cancers [1]. Like every DNA repair process, the success of MMR depends on two essential steps: lesion detection and removal. MMR is unique in two aspects. The targets of repair are normal rather than damaged nucleotides, and nucleotide removal has to be specific to the newly synthesized daughter strand and not the template. E. coli MMR is the best understood and has been fully reconstituted in vitro using a dozen or so purified proteins [2,3]. Recognition of a normal nucleotide that fails to make a Watson-Crick base pair in a DNA duplex is accomplished by the MutS protein. The strand specificity of MMR in E. coli is conferred by a sequence and methylation specific endonuclease MutH, which makes an incision (nick) 5′ to a GATC sequence in the unmethylated daughter strand. The GATC sequence may be several hundred base pairs from the mismatch on either the 5′ or 3′ side (Fig. 1A). The direction of daughter strand removal is strongly biased towards the shorter track between the nick and mismatch in a circular genome. The degradation of hundreds to a thousand nucleotides is carried out by one of several exonucleases with 5′ → 3′ or 3′-gt; 5′ polarity in the presence of the UvrD helicase and the single strand binding protein SSB. An additional central player in MMR is MutL, a molecular matchmaker that mediates proteinprotein interactions and coordinates mismatch recognition with strand incision and degradation (Fig. 1A).The MMR pathway is conserved from E. coli to humans [3]. The essential proteins known to specialize in human MMR are so far limited to MutS and MutL homologs and a 5′ → 3′ exonuclease, ExoI. Eukaryotic MutS and MutL are heterodimers of homologous subunits as opposed to homodimers in bacteria. Human MutSα consists of MSH2 and MSH6, and MutLα consists of MLH1 and PMS2. A strand-specific endonuclease that nicks the daughter strand, like MutH in E. coli, is missing in all eukaryotes and in many bacteria. It is thusCorrespondence and request for material should be addressed to W.Y. e-mail: Wei.Yang@nih.gov phone: (301), 402-4645, fax: (301) 496-0201. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its...

Bound Nucleotide Controls the Endonuclease Activity of Mismatch Repair Enzyme MutL

“…In particular, the suppression of nonspecific digestion of homoduplex by binding of ATP is emphasized. This result may answer the question how cells regulate the endonuclease activity of MutL (13).…”

Structure and mechanism for DNA lesion recognition

“…Crystal structures of Taq, E. coli and human MutS complexed with a variety of unpaired and mispaired bases [15][16][17][18] reveal how MutS recognizes a mismatch surrounded by random sequence. Biochemical studies of interactions between MutS and mismatch DNA and MutS with downstream effector proteins in E. coli and humans further illuminate the mechanism for MMR specificity [19].…”

Structural Features and Functional Dependency on β-Clamp Define Distinct Subfamilies of Bacterial Mismatch Repair Endonuclease MutL

“…MutL endonucleases have two domains: the N-terminal ATPase and the C-terminal endonuclease domains (15,16,(23)(24)(25). Crystallographic analyses on the C-terminal domain (CTD) of eukaryotic and bacterial MutL endonuclease have shown that the domain is composed of regulatory and dimerization subdomains (11, 26 -28).…”