In human cells, hMLH1, hMLH3, hPMS1 and hPMS2 are four recognised and distinctive homologues of MutL, an essential component of the bacterial DNA mismatch repair (MMR) system. The hMLH1 protein forms three different heterodimers with one of the other MutL homologues. As a first step towards functional analysis of these molecules, we determined the interacting domains of each heterodimer and tried to understand their common features. Using a yeast two-hybrid assay, we show that these MutL homologues can form heterodimers by interacting with the same amino acid residues of hMLH1, residues 492-742. In contrast, three hMLH1 partners, hMLH3, hPMS1 and hPMS2 contain the 36 homologous amino acid residues that interact strongly with hMLH1. Contrary to the previous studies, these homologous residues reside at the N-terminal regions of three subdomains conserved in MutL homologues in many species. Interestingly, these residues in hPMS2 and hMLH3 may form coiled-coil structures as predicted by the MULTICOIL program. Furthermore, we show that there is competition for the interacting domain in hMLH1 among the three other MutL homologues. Therefore, the quantitative balance of these three MutL heterodimers may be important in their functions.

Epigenetic silencing through methyl-CpG (mCpG) is implicated in many biological patterns such as genome imprinting, X chromosome inactivation, and cancer development. In this process, the mCpG binding domain (MBD) proteins play an essential role in transmitting epigenetic information to downstream regulatory proteins. Among the five MBD proteins identified so far, MBD4 has been the only exception; it has long been thought to be a DNA repair protein. Herein we demonstrate that MBD4 has the ability to repress transcription through mCpG. Transcriptional repression by the MBD4 is histone deacetylase (HDAC) dependent, and MBD4 directly binds to Sin3A and HDAC1 at three central regions that overlap transcriptional repression domains. Furthermore, a chromatin immunoprecipitation assay clearly shows that MBD4 binds to hypermethylated promoters of the p16INK4a and hMLH1 genes. These results suggest that MBD4 is one of the essential components involved in epigenetic silencing in cancer and its repair activity is necessary for the maintenance of hypermethylated promoters.

Anthocyanin O-methyltransferase (OMT) is one of the key enzymes for anthocyanin modification and flower pigmentation. We previously bred a novel red-purple-flowered fragrant cyclamen (KMrp) from the purple-flowered fragrant cyclamen 'Kaori-no-mai' (KM) by ion-beam irradiation. Since the major anthocyanins in KMrp and KM petals were delphinidin 3,5-diglucoside and malvidin 3,5-diglucoside, respectively, inactivation of a methylation step in the anthocyanin biosynthetic pathway was indicated in KMrp. We isolated and compared OMT genes expressed in KM and KMrp petals. RT-PCR analysis revealed that CkmOMT2 was expressed in the petals of KM but not in KMrp. Three additional CkmOMTs with identical sequences were expressed in petals of both KM and KMrp. Genomic PCR analysis revealed that CkmOMT2 was not amplified from the KMrp genome, indicating that ion-beam irradiation caused a loss of the entire CkmOMT2 region in KMrp. In vitro enzyme assay demonstrated that CkmOMT2 catalyzes the 3' or 3',5' O-methylation of the B-ring of anthocyanin substrates. These results suggest that CkmOMT2 is functional for anthocyanin methylation, and defective expression of CkmOMT2 is responsible for changes in anthocyanin composition and flower coloration in KMrp.

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The human PMS2 gene encodes one of the bacterial mutL homologs that is associated with hereditary nonpolyposis colorectal cancer (HNPCC). One of the interesting features of the hPMS2 gene is that it is part of a multiple gene family which is localized on chromosome bands 7p22, 7p12-p13, 7q11, and 7q22. Here we report four newly identified hPMS2-like (PMS2L) genes. All four novel members of the PMS2L gene family encode relatively short polypeptides composed of the amino-terminal portion of hPMS2 and are expressed ubiquitously except in the heart. To clarify whether the PMS2L polypeptides contribute to the DNA mismatch repair (MMR) pathway through an interaction with hMLH1, we have performed a yeast two-hybrid assay and an immunoprecipitation study using an hPMS2 mutant cell line, HEC-1-A. Our results clearly indicate that hMLH1 does not interact with two representative PMS2Ls, whereas the carboxyl-terminal portion of hPMS2, not the amino-terminal portion, does interact with hMLH1. Thus, PMS2Ls are not likely to participate in the MMR pathway through association with hMLH1; they must play some other roles in the living cells.