The oxygenase Jmjd6--a case study in conflicting assignments.

The Jumonji domain-containing protein 6 (Jmjd6) is a member of the superfamily of non-haem iron(II) and 2-oxoglutarate (2OG)-dependent oxygenases; it plays an important developmental role in higher animals. Jmjd6 was initially assigned a role as the phosphatidylserine receptor responsible for engulfment of apoptotic cells but this now seems unlikely. Jmjd6 has been shown to be a nuclear localized protein with a JmjC domain comprising a distorted double-stranded β-helical structure characteristic of the 2OG-dependent oxygenases. Jmjd6 was subsequently assigned a role in catalysing N-methyl-arginine residue demethylation on the N-terminus of the human histones H3 and H4; however, this function is also subject to conflicting reports. Jmjd6 does catalyse 2OG-dependent C-5 hydroxylation of lysine residues in mRNA splicing-regulatory proteins and histones; there is also accumulating evidence that Jmjd6 plays a role in splicing (potentially in an iron- and oxygen-dependent manner) as well as in other processes regulating gene expression, including transcriptional pause release. Moreover, a link with tumour progression has been suggested. In the present review we look at biochemical, structural and cellular work on Jmjd6, highlighting areas of controversy and consensus.

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