Mark C. Hall

Assistant Professor of Biochemistry
214 Biochemistry, (765) 494-0714
Ph.D. University of North Carolina, Chapel Hill, 1998
mchall@purdue.edu

Investigators: 
Graduate Students:
Eunyoung Choi
Juan Martinez

Area:  Regulation of the cell cycle by ubiquitin-dependent proteolysis; protein mass spectrometry

Background.  The cell division cycle in eukaryotic organisms is a complex series of strictly regulated events that ensures 1) that the genome is replicated accurately and 2) that complete copies of the genome are faithfully distributed to the daughter cells.  Cancer ultimately results from loss of control over the cell division cycle, and therefore a clear understanding of the molecular processes that dictate cell cycle progression is essential to a fundamental understanding of carcinogenesis.  In all eukaryotic cells the sequential stages of the cell cycle are regulated by two primary mechanisms:  reversible protein phosphorylation, and ubiquitin-dependent proteolysis.  These processes are interdependent, and together they drive the cell cycle forward with the proper order and timing.

Our lab is interested in how ubiquitin-dependent proteolysis directed by the anaphase-promoting complex (APC) is used to regulate mitotic and meiotic events in the budding yeast, Saccharomyces cerevisiae.  Budding yeast are easy to work with and manipulate genetically, and therefore make a highly attractive and suitable model organism for studying the evolutionarily conserved cell cycle and understanding its relationship to cancer in humans.  The APC is a large, highly conserved, multisubunit enzyme that catalyzes the polyubiquitylation of specific substrate proteins at specific points during the cell cycle.  The polyubiquitin tags serve as markers for substrates to be destroyed by the proteasome.  By this mechanism, the cell cycle progresses from one stage to the next by the selective and irreversible elimination of inhibitory proteins. The exquisite selectivity and timing of APC activity is a fascinating area of research and a major topic of focus in our lab.

Phosphoregulation of APC activators.  Although the APC is constitutively present during the cell cycle, its activity is tightly regulated.  A major target of APC regulation is a family of activator proteins that are essential for APC activity.  These activators are not stable components of the APC but instead interact with it at specific times to promote APC activity, presumably by recruiting specific substrates that are not recognized by the APC alone.  There are three known activators in budding yeast (Cdc20, Cdh1, and Ama1) and we are interested in studying how phosphorylation regulates their interaction with the APC and its substrates, and thereby controls APC activity during the cell cycle.  Our efforts include identification of in vivo phosphorylation sites and the kinases responsible for them, understanding how phosphorylation sites change during the cell cycle, and using site-specific mutations to determine the role of phosphorylation in APC regulation.

APC activity during meiosis.  Meiosis is a specialized cell division cycle in which two consecutive rounds of chromosome segregation occur without an intervening S phase, resulting in reduction of the chromosome content from two sets to one and the formation of haploid gametes.  The APC is an essential regulatory element in meiosis, just as it is in mitosis.  Several lines of evidence suggest that the APC performs additional regulatory functions that are specific to meiosis.  First, expression of virtually all APC subunits is dramatically induced during yeast meiosis.  Second, a meiosis-specific APC activator protein and a meiosis-specific APC regulatory kinase have been identified.  Third, we recently identified two novel subunits of the yeast APC associated with meiosis-specific phenotypes.  Finally, ubiquitin-dependent proteolysis is an attractive mechanism for a cell to eliminate meiotic proteins once their meiotic functions have been performed or once the specialized meiotic program has been completed.  However, relatively little is known about meiosis-specific APC functions, and no meiosis-specific substrates of the APC have been identified. A second major goal of our lab is to identify meiotic substrates of the APC and understand the role of the meiosis-specific activator Ama1 in the selection of these substrates.  We would like to understand how Ama1 interacts with the APC and how phosphorylation of Ama1 and APC regulates this interaction.  These studies will hopefully provide useful insights into the fundamental mechanisms of meiotic regulation in eukaryotes.