Mona T. Norcum, Ph.D.

Graduated in 1982 at the University of California-Los Angeles (Mentor:  Dohn G. Glitz).
Postdoctoral Studies with William R. Clark at the Molecular Biology Institute, UCLA (1982-1984) and
with Gerald M. Carlson at the University of Mississippi Medical Center (1984-1986)
Instructor, Department of Biochemistry, UMMC (1986-1987)
Assistant Professor, Department of Biochemistry, UMMC (1987-1993)
Associate Professor, Department of Biochemistry, UMMC (1993-1999)
Professor, Department of Biochemistry, UMMC (1999-Present)

Currently, our major project is the characterization of multienzyme complexes of aminoacyl-tRNA synthetases. This family of enzymes catalyzes a primary event of protein synthesis: the covalent attachment of each amino acid to its specific transfer RNA. In multicellular eukaryotes, several of these enzymes are isolated in a high molecular mass complex, which is well definedwell-defined biochemically but not structurally. Based on biochemical data, wWee have developed a three domain working model of the multisynthetase complex which we are refining using the above methods. It is particularly intriguing that one of the non-synthetase polypeptides within the particle (p43) is the precursor of an inflammatory cytokine (EMAPII) that is released by caspases in response to apoptotic signals. We have recently shown that this protein occupies a central location within the multisynthetase complex. Thus, it is suitably placed to control the structure and function of this essential component of protein biosynthesis. Using computational microscopy, we have now identified general sites of tRNA binding using oxidized total yeast tRNA, the active site of LeuRS using Nanogold- labeled tRNAleu, and the C-termini of p43-his6 using Nanogold-NiNTA. This allows us to translate our working model into three-dimensions. These results provide the first ideas of the spatial arrangement of proteins, enzyme active sites and multiple substrate binding sites within the multi-aaRS structure. In effect, we now have specific insight into how this unique particle functions as an ‘aminoacylation machine’.

Other projects include exploration of the structure and function of the complex of valyl-tRNA synthetase with elongation factor 1H. This intriguing particle carries out two successive steps of the protein biosynthesis process. We are also investigating changes in the multisynthetase particle and p43 in response to metabolic stressors. We are currently investigating these possibilities. Future studies will focus on the physical interaction of the multisynthetase complex with other polypeptides and macromolecules, such as elongation factors and ribosomes. This information is intrinsic to a complete understanding of the overall protein biosynthetic process as well as the principles underlying intracellular organization. These studies are also of medical importance in designing new antimicrobials, as well as understanding the role of aminoacyl-tRNA synthetases in post-ischemic suppression of protein synthesis and in rheumatic autoimmune disease.

Current collaborative projects include: ultrastructure and oligomerization state of meprin, a zinc metalloprotienase; overall structure of and component localization within complexes of Rep68 and Rep 78 helicases with DNA, and shape and oligomeric state of T4 helicase/primase complexes. All of tThese studies and several collaborative projects utilize the UMMC Electron Microscopy Facility, which is under the direction of Dr. Norcum. This is based around a LEO912AB transmission electron microscope, which was installed in summer of 1998. The microscope is equipped with a LaB6 crystalline electron source, integrated Omega energy filter, computerized tilt stage, Oxford cryoholder, and minimum-dose focusing software. Data collection is via film or a computer-controlled image intensifying camera. Ancillary instrumentation for computational analysis of biological structures includes twoan Octane Silicon Graphics Workstationscomputer, an Imacon film AGFA Duoscan flatbed scanner, and Kodak dye sublimation printer.

All of these studies utilize the UMMC Electron Microscopy Facility, which is under the direction of Dr. Norcum. This is based around a LEO912AB transmission electron microscope, which was installed in summer of 1998. The microscope is equipped with a LaB₆ crystalline electron source, integrated Omega energy filter, computerized tilt stage, Oxford cryoholder, and minimum-dose focusing software. Data collection is via film or a computer-controlled image intensifying camera. Ancillary instrumentation for computational analysis of biological structures includes an Octane Silicon Graphics computer, AGFA Duoscan flatbed scanner, and Kodak dye sublimation printer.

Wolfe, C.L., Warrington, J.A., Treadwell, L., and NORCUM, M.T. A Three-Dimensional Working Model of the Multisynthetase Complex based on Placements of tRNA and Proteins using Computational Microscopy. J. Biol. Chem., submitted March 2005.

NORCUM, M.T., Warrington, J.A., Spiering, M., Ishmael, F.T., Trakselis, M.A., and Benkovic, S.J. 2005. Architecture of the Bacteriophage T4 Primosome: Electron Microscopy Studies of Helicase (gp41) and Primase (gp61). Proc. Natl. Acad. Sci. 10: 3623-3626.

Wolfe, C.L., Warrington, J.A., Davis, S., Green, S., and NORCUM, M.T. 2003. Isolation and Characterization of Human Nuclear and Cytosolic Multisynthetase Complexes and the Intracellular Distribution of p43/EMAPII. Prot. Sci. 12: 2282-2290.

Bertenshaw, G.B., NORCUM, M.T. and Bond, J.S. 2003. Structure of Homo- and Hetero-oligomeric Meprin Metalloproteinases: Dimers, Tetramers, and High Molecular Mass Multimers. J. Biol. Chem. 278:2522-2532.

Ishmael, F.T., Trakselis, M.A., NORCUM, M.T. and Benkovic, S.J. 2002. Architecture and Assembly of the Bacteriophage T4 Primosome: A Ring Trilogy. Science, in revision.

Norcum, M.T. and Boisset, N. 2002. Three-dimensional architecture of the eukaryotic multisynthetase complex determined from negatively stained and cryo electron micrographs. FEBS Lett. 512: 298-302.

Traxler, K.W., Norcum, M.T., Hainfeld, J.F., Carlson, G.M. 2001. Direct visualization of the calmodulin subunit of phosphorylase kinase via electron microscopy following subunit exchange. J. Struct. Biol. 135: 231-238.

Norcum, M.T. and Warrington, J.A. 2000. The cytokine portion of p43 occupies a central position within the eukaryotic multisynthetase complex. J Biol Chem Accelerated Publication 275:17921-17924.

Norcum, M.T. and Dignam, J.D. 1999. Immunoelectron microscopic localization of glutamyl-/prolyl-tRNA synthetase within the eukaryotic multisynthetase complex. J. Biol. Chem. 274: 12205-12208.

Norcum, M.T. 1999. Ultrastructure of the eukaryotic multisynthetase complex derived from two-dimensional averaging and classification of negatively stained electron micrographs. FEBS Lett. 447: 217-222.

Norcum, M.T. and Warrington, J.A. 1998. Structural analysis of the multienzyme amino-acyl tRNA synthetase complex: a three domain model based on reversable crosslinking. Prot. Sci. 7: 79-87.