Erica S. Johnson, PhD

Contact Dr. Johnson

233 South 10th Street
BLSB 231
Philadelphia, PA 19107

(215) 503-4616
(215) 503-5393 fax

Medical School

PhD, MIT, Cambridge, MA - 1992

Expertise and Research Interests

Covalent modifications of proteins play critical roles in most cellular processes through their ability to cause rapid and reversible changes in the functions of pre-existing proteins, multi-protein complexes and subcellular structures. This type of mechanism for regulating protein function is especially important in signal transduction pathways, in transcriptional regulation, and in the cell cycle.

We are using the budding yeast Saccharomyces cerevisiae to study a modification that involves attachment of the small ubiquitin-related protein SUMO to other proteins. SUMO (also called Smt3 in yeast) is found in all eukaryotes, and it is becoming clear that SUMO conjugation plays an important role in cell cycle control and proliferation in both yeast and mammalian cells. However, very little is known about the the molecular mechanism of SUMO function. S. cerevisiae is a powerful system for studying conserved processes such as SUMO conjugation because it can be easily manipulated in genetic, biochemical and cell biological studies, and the lessons learned are likely to be applicable to more complex organisms.

We are interested in two major areas related to SUMO conjugation. The first is the identification and characterization of the enzymes required for attaching SUMO to other proteins. We have identified the enzymes that carry out all three steps of the SUMO conjugation reaction. The enzymes responsible for the third step, so-called E3s or SUMO ligases, are likely to be the critical factors in selecting substrates for SUMO attachment and for regulating this process. We are in the process of investigating how E3s bind their substrates and how the cell uses E3s to regulate SUMO conjugation.

Our second major interest is in determining the biological function of SUMO conjugation in yeast. We are attempting to find proteins that are modified by SUMO using a combination of approaches. One strategy is to purify proteins bearing SUMO, followed by identification of the proteins by mass spectrometry. The other is to isolate yeast mutants that interact functionally with the SUMO pathway. These mutants could lead to identification of new substrates as well as other proteins that participate in SUMO-mediated processes.


Biochemistry; Sumo; Ubiquitin-Like Proteins; Yeast