Roles of the hydroxy group of tyrosine in crystal structures of Sulfurisphaera tokodaiiO6-methylguanine-DNA methyltransferase

The DNA Alkyltransferase Family of DNA Repair Proteins: Common Mechanisms, Diverse Functions

DNA alkyltransferase and alkyltransferase-like family proteins are responsible for the repair of highly mutagenic and cytotoxic O6-alkylguanine and O4-alkylthymine bases in DNA. Their mechanism involves binding to the damaged DNA and flipping the base out of the DNA helix into the active site pocket in the protein. Alkyltransferases then directly and irreversibly transfer the alkyl group from the base to the active site cysteine residue. In contrast, alkyltransferase-like proteins recruit nucleotide excision repair components for O6-alkylguanine elimination. One or more of these proteins are found in all kingdoms of life, and where this has been determined, their overall DNA repair mechanism is strictly conserved between organisms. Nevertheless, between species, subtle as well as more extensive differences that affect target lesion preferences and/or introduce additional protein functions have evolved. Examining these differences and their functional consequences is intricately entwined with understanding the details of their DNA repair mechanism(s) and their biological roles. In this review, we will present and discuss various aspects of the current status of knowledge on this intriguing protein family.

Archaeal DNA alkylation repair conducted by DNA glycosylase and methyltransferase

Alkylated bases in DNA created in the presence of endogenous and exogenous alkylating agents are either cytotoxic or mutagenic, or both to a cell. Currently, cells have evolved several strategies for repairing alkylated base. One strategy is a base excision repair process triggered by a specific DNA glycosylase that is used for the repair of the cytotoxic 3-methyladenine. Additionally, the cytotoxic and mutagenic O⁶-methylguanine (O⁶-meG) is corrected by O⁶-methylguanine methyltransferase (MGMT) via directly transferring the methyl group in the lesion to a specific cysteine in this protein. Furthermore, oxidative DNA demethylation catalyzed by DNA dioxygenase is utilized for repairing the cytotoxic 3-methylcytosine (3-meC) and 1-methyladenine (1-meA) in a direct reversal manner. As the third domain of life, Archaea possess 3-methyladenine DNA glycosylase II (AlkA) and MGMT, but no DNA dioxygenase homologue responsible for oxidative demethylation. Herein, we summarize recent progress in structural and biochemical properties of archaeal AlkA and MGMT to gain a better understanding of archaeal DNA alkylation repair, focusing on similarities and differences between the proteins from different archaeal species and between these archaeal proteins and their bacterial and eukaryotic relatives. To our knowledge, it is the first review on archaeal DNA alkylation repair conducted by DNA glycosylase and methyltransferase. KEY POINTS: • Archaeal MGMT plays an essential role in the repair of O ⁶ -meG • Archaeal AlkA can repair 3-meC and 1-meA.