Kevin D Sarge, PhD

1.  Protein sumoylation and its role in human diseases.

         

           Sumoylation, the covalent attachment of Small Ubiquitin-like Modifier (SUMO) proteins to specific lysine residues in target proteins, regulates many aspects of normal protein function,  including subcellular localization, protein partnering, and transcription factor transactivation [1-6].  Cells express three major SUMO paralogs, SUMO-1, SUMO-2, and SUMO-3, with SUMO-2 and SUMO-3 being much more similar to each other than to SUMO-1.  The SUMO E2 enzyme, called UBC9, attaches the SUMO proteins to a lysine or lysines in the target protein that are typically, but not always, found within the consensus sequence ?KXE (? represents hydrophobic amino acids).  SUMO E3 proteins stimulate protein sumoylation by associating with both UBC9 (the E2 enzyme) and substrates to increase the efficiency of the modification reaction.

           Our lab recently discovered that a protein that plays an important role in nuclear architecture, lamin A, is sumoylated and that this modification is required for the proper localization of this protein to the nuclear periphery.  We also found that two mutations in lamin A that cause a heart disease called cardiomyopathy, mutations that change the conserved glutamic acid residue immediately adjacent to the sumoylation site of lamin A, result in this protein being unable to be sumoylated.  This is the first known example of a human disease that is caused by a mutation that prevents sumoylation of a protein.  This work also suggests that interventions that increase sumoylation of the lamin A protein could provide potential treatments for this type of heart disease.

            In another project in our lab we found that the amyloid precursor protein (APP) is sumoylated at two lysine residues that are immediately adjacent to the site where this protein is cleaved by the b-secretase, an event that leads to generation of the Aß peptide thought by many to be involved in causing Alzheimer’s disease.  Importantly, preventing APP sumoylation leads to higher levels of Aß protein aggregates, suggesting that this modification inhibits Aß formation. Further, up-regulating cellular levels of the SUMO E2 enzyme UBC9 resulted in elevated APP sumoylation and decreased Aß protein aggregate levels.  These results suggest that interventions that alter APP sumoylation could represent a potential new approach for combating Alzheimer’s disease.





2.  Gene Bookmarking: Epigenetic process that preserves the “memory” of active gene expression.



           Several years ago we found that a protein called HSF2 functions to prevent the promoter of the gene encoding the critical stress-protective protein hsp70i from being compacted during mitosis, in contrast to most of the genomic DNA which is tightly compacted.  This lack of compaction is important because it provides cells with the ability to turn on transcription of the hsp70i gene even in early G1 phase if a stress occurs.  Otherwise, the cell would be unable to protect itself until it could de-compact the promoter region.  HSF2 prevents compaction of the hsp70i promoter by binding to its HSEs at the beginning of mitosis, recruiting protein phosphatase 2A, and interacting with a subunit of the enzyme called condensin that is important for mitotic DNA compaction, so that the HSF2-associated PP2A can dephosphorylate and inactivate the condensin to prevent compaction of this specific region of the chromosome.

           Our studies on HSF2-mediated bookmarking of the hsp70 promoter led us into a second major area of investigation for our lab.  This area is the study of the epigenetic mechanism called gene bookmarking, which functions to precisely transmit the “memory” of what genes were active prior to entry into mitosis to their daughter cells, despite the fact that transcription ceases and chromosomes are highly compacted during this stage of the cell cycle.  This gene bookmarking is essential for ensuring the faithful transmission of gene expression patterns, and thus phenotype, down cell lineages, without which multicellular organisms could not exist.  Knowledge of the gene bookmarking mechanism could also lead to advances in generating stem cells from adult cells and cloning animals via somatic cell nuclear transfer, because failure to properly reprogram gene bookmarking is believed to be a key barrier limiting the success of both of these processes.

           In a recent paper in Nature Cell Biology, we showed that the general transcription factor called TATA-binding protein (TBP) plays a critical role in gene bookmarking of formerly active genes.  We found that during mitosis TATA-binding protein (TBP), which remains bound to DNA during mitosis, recruits PP2A and also interacts with condensin to allow efficient dephosphorylation/inactivation of condensin near these promoters to inhibit their compaction.  These results suggest that TBP is involved not only in gene transcription during interphase but also in preserving the memory of gene activity through mitosis to daughter cells.