Bert and Natalie Vallee Professor of Molecular Pathology
Harvard Medical School
NRB-940E
77 Ave Louis Pasteur
Boston, MA 02115
PH: 617-432-6590
FX: 617/277-6591
wade_harper@hms.harvard.edu

Lab Website

Research Interests

My laboratory is interested in the problem of how the cell division cycle is controlled. Progression through the cell cycle is regulated by a family of cyclin-dependent kinases (Cdks) composed of a catalytic Cdk subunit and a regulatory cyclin subunit. These kinases act to promote DNA replication and mitotic progression. These kinases are negatively regulated by Cdk inhibitors, phosphorylation, and ubiquitin-mediated proteolysis.

One area of interest concerns how the G1 cyclins, cyclin E and cyclin D, control S-phase and how their activities are regulated. In previous work, we have determined that cyclin E can function both upstream and downstream of the Retinoblastoma protein Rb to control S-phase entry, whereas an important role of cyclin D/Cdk4 is to phosphorylation Rb and its homolog p107. Recently, we have used expression cloning techniques to identify cyclin E targets. One protein, p220NPAT, is a central regulator of S-phase. p220 is localized in nuclear organelles called Cajal Bodies. There are two p220-positive Cajal Bodies in G1 cells that are physically attached to histone gene clusters on chromosome 6. As cells enter S-phase, an additional pair of p220-containing Cajal Bodies assemble at histone gene clusters on chromosome 1. p220 expression promotes S-phase progression and activation of histone gene expression and these activities are enhanced by cyclin E/Cdk2. We recently generated human somatic cells that contain a conditional allele of p220 using homologous targeting. We have found that p220 is essential for cell division and is required for S-phase entry. In addition, p220 is required for histone transcription, which is normally activated as cells enter S-phase. Future studies are concentrated on the identification of p220 interacting proteins.

Another major interest of the lab is in the area of protein ubiquitination. Previously, my lab together with the Elledge lab, elucidated the SCF ubiquitin ligase pathway. SCF complexes are composed of Skp1, Cul1, Rbx1, and a member of the F-box family of proteins. F-box proteins function as substrate specific receptors for ubiquitination targets, and frequently interact with targets in a phosphorylation-dependent manner. There are more than 70 F-box proteins in the human genome. We demonstrated that SCFCdc4 is responsible for ubiquitination of the Cdk inhibitor Sic1, SCFGrr1 is responsible for Cln2 (G1 cyclin) ubiquitination, and SCF?-TRCP is responsible for IkBa ubiquitination. More recently, we identified Fbw7 as the F-box for cyclin E. Current experiments seek to use genomic and proteomic approaches to understand the function of F-box proteins. In addition, we have recently identified a large family of BTB-domain containing proteins (with nearly 200 members in humans) which function as adaptor proteins for the Cul3 ubiquitin ligase. The available data indicate that these BTB proteins serve a substrate spefic adaptors for the Cul-3 ubiquitin ligase. Current efforts are placed on identification of the pathways that these genes control in mammals and C. elegans.

Publications

Ma, T., et al. (2000) Cell-cycle regulated phosphorylation of p220NPAT by cyclin E/Cdk2 in Cajal Bodies promotes histone gene transcription. Genes and Development 14, 2298-2313.

Wang, Y., et al. (2003) Exit from exit: Resetting the cell cycle through AMEN inhibition of G-protein signaling. Cell, 112: 697-709.

 Zheng, N., et al.(2002) Structure of the Cul1-Rbx1-Skp1-F boxSkp2 SCF Ubiquitin Ligase Complex. Nature 416, 703-709.