The PHD finger of Spp1 mediates histone modification cross-talk

Binding of the Spp1 PHD finger to histone H3K4me3 is sensitive to adjacent post-translational modifications in the histone tail. This commentary discusses the findings of He and colleagues [Biochem. J.476, 1957–1973] which show that the PHD finger binds to H3K4me3 in a selective manner which is conserved in the Saccharomyces pombe and mammalian orthologues of Spp1.

Structural basis for histone H3K4me3 recognition by the N-terminal domain of the PHD finger protein Spp1

Saccharomyces cerevisiae Spp1, a plant homeodomain (PHD) finger containing protein, is a critical subunit of the histone H3K4 methyltransferase complex of proteins associated with Set1 (COMPASS). The chromatin binding affinity of the PHD finger of Spp1 has been proposed to modulate COMPASS activity. During meiosis, Spp1 plays another role in promoting programmed double-strand break (DSB) formation by binding H3K4me3 via its PHD finger and interacting with a DSB protein, Mer2. However, how the Spp1 PHD finger performs site-specific readout of H3K4me3 is still not fully understood. In the present study, we determined the crystal structure of the highly conserved Spp1 N-terminal domain (Sc_Spp1NTD) in complex with the H3K4me3 peptide. The structure shows that Sc_Spp1NTD comprises a PHD finger responsible for methylated H3K4 recognition and a C3H-type zinc finger necessary to ensure the overall structural stability. Our isothermal titration calorimetry results show that binding of H3K4me3 to Sc_Spp1NTD is mildly inhibited by H3R2 methylation, weakened by H3T6 phosphorylation, and abrogated by H3T3 phosphorylation. This histone modification cross-talk, which is conserved in the Saccharomyces pombe and mammalian orthologs of Sc_Spp1 in vitro, can be rationalized structurally and might contribute to the roles of Spp1 in COMPASS activity regulation and meiotic recombination.

A transcription factor primes the condensin pump

Chromosome condensation is regulated by the condensin complex but whether this process is subject to transcriptional control is poorly understood. In this issue, Schiklenk et al. (2018. J. Cell Biol. https://doi.org/10.1083/jcb.201711097) reveal that the transcription factor Zas1 mediates timely chromosome condensation and promotes transcription of several genes in Saccharomyces pombe, including the condensin subunit Cnd1.

The epigenetic regulation of autonomous replicons

The discovery of autonomous replicating sequences (ARSs) inSaccharomyces cerevisiaein 1979 was considered a milestone in unraveling the regulation of replication in eukaryotic cells. However, shortly afterwards it became obvious that inSaccharomyces pombeand all other higher organisms ARSs were not sufficient to initiate independent replication. Understanding the mechanisms of replication is a major challenge in modern cell biology and is also a prerequisite to developing application-oriented autonomous replicons for gene therapeutic treatments. This review will focus on the development of non-viral episomal vectors, their use in gene therapeutic applications and our current knowledge about their epigenetic regulation.

Expression of human liver epoxide hydrolase in Saccharomyces pombe

Human liver microsomal epoxide hydrolase cDNA was inserted into the yeast expression vector pEVP11. The resulting recombinant plasmid was introduced into Saccharomyces pombe. The epoxide hydrolase protein and enzymic activity was subsequently expressed and identified in the 105,000 g pellet after centrifugal fractionation of homogenized yeast cells. This method will provide a useful source of human liver epoxide hydrolase, avoiding the problems of obtaining human tissue.