Putting a finger on histidine methylation

Specialized enzymes add methyl groups to the nitrogens of the amino acid histidine, altering the chemical properties of its imidazole ring and, in turn, the function of the modified (poly)peptide. In this issue of Genes & Development, Shimazu and colleagues (pp. 724-742) make the remarkable discovery that CARNMT1 acts as a dual-specificity histidine methyltransferase, modifying both the small-molecule dipeptide carnosine and a set of proteins, predominantly within RNA-binding C3H zinc finger (C3H ZF) motifs. As a result, CARNMT1 modulates the activity of its protein targets to affect RNA processing and metabolism, ultimately contributing an essential function during mammalian development.

Histidine N1-position-specific methyltransferase CARNMT1 targets C3H zinc finger proteins and modulates RNA metabolism

Histidine (His) residues are methylated in various proteins, but their roles and regulation mechanisms remain unknown. Here, we show that carnosine N-methyltransferase 1 (CARNMT1), a known His methyltransferase of dipeptide carnosine (βAla-His), is a major His N1-position-specific methyltransferase. We found that 52 His sites in 20 proteins underwent CARNMT1-mediated methylation. The consensus methylation site for CARNMT1 was identified as Cx(F/Y)xH, a C3H zinc finger (C3H ZF) motif. CARNMT1-deficient and catalytically inactive mutant mice showed embryonic lethality. Among the CARNMT1 target C3H ZF proteins, RNA degradation mediated by Roquin and tristetraprolin (TTP) was affected by CARNMT1 and its enzymatic activity. Furthermore, the recognition of the 3' splice site of the CARNMT1 target C3H ZF protein U2AF1 was perturbed, and pre-mRNA alternative splicing (AS) was affected by CARNMT1 deficiency. These findings indicate that CARNMT1-mediated protein His methylation, which is essential for embryogenesis, plays roles in diverse aspects of RNA metabolism by targeting C3H ZF-type RNA-binding proteins and modulating their functions, including pre-mRNA AS and mRNA degradation regulation.

Biosynthesis of Carnosine and Related Dipeptides in Vertebrates

Carnosine (β-alanyl-L-histidine) and its methylated derivatives: anserine (β-alanyl-Nπ- methyl-L-histidine) and balenine (β-alanyl-Nτ-methyl-L-histidine) are abundant constituents of excitable tissues of vertebrates. While carnosine and anserine are present at high concentrations and in variable proportions in skeletal muscle and brain of most vertebrates, balenine appears to be rather more abundant in marine mammals and certain reptilian species. Since the discovery of these compounds at the beginning of 20th century, numerous studies have been devoted to identification of the biochemical and physiological properties of carnosine and related dipeptides. These led to the discovery of the pHbuffering, metal-chelation and antioxidant, capabilities of carnosine and anserine, although no definitive ideas concerning their physiological role has yet been formulated. Only recently the molecular identities of the enzymes catalyzing synthesis of carnosine (carnosine synthase, EC 6.3.2.11) and anserine (carnosine N-methyltransferase, EC 2.1.1.22) have been elucidated, which has given a new insight into their metabolism in vertebrates. These findings have opened new research areas and provide authentic opportunities for understanding the biological function of these "enigmatic" dipeptides. This review aims to summarize recent advances in our knowledge concerning enzymes responsible for the biosynthesis of carnosine and related dipeptides and to evaluate their importance in vertebrate physiology.