The Tudor Domain of the PHD Finger Protein 1 Is a Dual Reader of Lysine Trimethylation at Lysine 36 of Histone H3 and Lysine 27 of Histone Variant H3t

PHF1 associates with the Polycomb repressive complex 2 and it was demonstrated to stimulate its H3K27-trimethylation activity. We studied the interaction of the PHF1 Tudor domain with modified histone peptides and found that it recognizes H3K36me3 and H3tK27me3 (on the histone variant H3t) and that it uses the same trimethyllysine binding pocket for the interaction with both peptides. Since both peptide sequences are very different, this result indicates that reading domains can have dual specificities. Sub-nuclear localization studies of full-length PHF1 in human HEK293 cells revealed that it co-localizes with K27me3, but not with K36me3, and that this co-localization depends on the trimethyllysine binding pocket indicating that K27me3 is an in vivo target for the PHF1 Tudor domain. Our data suggest that PHF1 binds to H3tK27me3 in human chromatin, and H3t has a more general role in Polycomb regulation.     

Toward the structure of presenilin/γ-secretase and presenilin homologs

Presenilin is the catalytic component of the γ-secretase complex, a membrane-embedded aspartyl protease that plays a central role in biology and in the pathogenesis of Alzheimer’s disease. Upon assembly with its three protein cofactors (nicastrin, Aph-1 and Pen-2), presenilin undergoes autoproteolysis into two subunits, each of which contributes one of the catalytic aspartates to the active site. A family of presenilin homologs, including signal peptide peptidase, possess proteolytic activity without the need for other protein factors, and these simpler intramembane aspartyl proteases have given insight into the action of presenilin within the γ-secretase complex. Cellular and molecular studies support a nine-transmembrane topology for presenilins and their homologs, and small-molecule inhibitors and cysteine scanning with crosslinking have suggested certain presenilin residues and regions that contribute to substrate recognition and handling. Identification of partial complexes has also offered clues to protein–protein interactions within the γ-secretase complex. Biophysical methods have allowed 3D views of the γ-secretase complex and presenilins. Most recently, the crystal structure of a microbial presenilin homolog has confirmed a nine-transmembrane topology and intramembranous location and proximity of the two conserved and essential aspartates. The crystal structure also provides a platform for the formulation of specific hypotheses regarding substrate interaction and catalysis as well as the pathogenic mechanism of Alzheimer-causing presenilin mutations. This article is part of a Special Issue entitled: Intramembrane Proteases.