The proteasome and epigenetics: zooming in on histone modifications

The proteasome is a structural complex of many proteins that degrades substrates marked by covalent linkage to ubiquitin. Many years of research has shown a role for ubiquitin-proteasome-mediated proteolysis in synaptic plasticity and memory mainly in degrading synaptic, cytoplasmic and nuclear proteins. Recent work indicates that the proteasome has wider proteolytic and non-proteolytic roles in processes such as histone modifications that affect synaptic plasticity and memory. In this review, we assess the evidence gathered from neuronal as well as non-neuronal cell types regarding the function of the proteasome in positive or negative regulation of posttranslational modifications of histones, such as acetylation, methylation and ubiquitination. We discuss the critical roles of the proteasome in clearing excess histone proteins in various cellular contexts and the possible non-proteolytic functions in regulating transcription of target genes. In addition, we summarize the current literature on diverse chromatin-remodeling machineries, such as histone acetyltransferases, deacetylates, methyltransferases and demethylases, as targets for proteasomal degradation across experimental models. Lastly, we provide a perspective on how proteasomal regulation of histone modifications may modulate synaptic plasticity in the nervous system.