Proliferating cell nuclear antigen is required for loading of the SMCX/KMD5C histone demethylase onto chromatin

In this study, we found that small interfering RNA-mediated knockdown of proliferating cell nuclear antigen (PCNA) resulted in the reduction of the chromatin-bound SMCX fraction. We identified a PCNA-interaction protein motif (PIP box) in the SMCX protein. Using site-directed mutagenesis, we found that the amino acids of the SMCX PIP box are involved in the association of SMCX with PCNA and its interaction with chromatin.Our data indicate that the intracellular trafficking of SMCX is controlled by its association with PCNA.Cellular DNA in eukaryotes is organized in chromatin, which contains a multitude of proteins. Histone proteins comprise a major structural component of chromatin. Histones carry diverse post-translational modifications, particularly on the lysines of their N-terminus. These modifications include methylation, acetylation, ubiquitylation and phosphorylation [1-4]. Histone modifications are involved in crucial cellular processes, including transcription, DNA replication, and the DNA-damage response [1,3,5]. As might be expected, chromatin and histones undergo major reorganization during DNA replication. After replication, the appropriate chromatin structure has to be restored in order to maintain the passage of information from mother to daughter cells.One of the major histone modifications is methylation of the lysine 4 residue of histone H3 (H3K4). Trimethylation of H3K4 is a hallmark of active transcription, and conversely, a loss of this modification represses transcription. Histone methylation is a very stable modification compared with, for example, acetylation. It had been long thought that this modification was irreversible [6]. This was supported by the fact that histone methyltransferases were identified, but their counterparts, histone demethylases, were not yet known. This situation changed with the first discovery of a histone demethylase, termed lysine-specific demethylase (LSD)1 [7]. LSD1 prefers double-methylated H3K4 as its substrate