You are cordially invited to a talk of the Edmond J. Safra Center for
Bioinformatics Distinguished Speaker Series.
The speaker is Prof. Eugene Shakhnovich, Chemistry and Chemical
Biology, Harvard University.
Title: "Bridging Scales in Biology: From Atoms to Populations"
Time: Tuesday, 27 March 2012, at 15:15 (refreshments from 15:00)
Place: Britania 06, Life Sciences Faculty
Abstract: In this presentation I will describe our efforts at understanding how molecular
properties of proteins determine fitness landscape of populations of carrier organisms.
Recent multi-scale evolutionary models, which assume certain relationship between
organismal fitness and stability of their proteins, have been successful in predicting such
biological phenomena as lethal mutagenesis (six mutations per genome per generation),
distributions of protein stabilities (‘’marginal’’ protein stability being a consequence of a
mutation-selection balance), correlation between evolutionary rates and abundances.
However, many of the underlying assumptions of these models have not been tested
experimentally. Our recent efforts aim to close this gap. We explored fitness landscape of
E.coli through controlled rational mutational genomic perturbations of expression level and
stability of an essential protein Dihydrofolate Reductase (DHFR). To that end we created
transgenic E.coli, which carry specified chromosomally incorporated mutations in the folA
gene encoding DHFR and also placed the folA gene under an IPTG controllable promoter,
making it possible to change the intracellular abundance of DHFR in a wide range. Using
competition essays, we measured how biological fitness depends on biophysical properties
of chromosomally incorporated mutant DHFR such as their abundance in the cytoplasm,
stability of its native state and folding intermediate, and catalytic activity. Mutant DHFR
proteins in a few strains aggregated rendering them nonviable but the majority exhibited
fitness higher than wild type at a growth temperature of 42oC. We found that mutational
destabilization of DHFR proteins in E. coli is counterbalanced by soluble oligomerization that
restores their structural stability and protects from aggregation. Further, we found that
protein homeostasis plays a defining role in sculpting fitness effect of mutations. In
particular, overexpression of GroEL as well as deletion of one of the proteases, Lon, resulted
in complete recovery of fitness of unviable strains. Further study, including in vitro essays of
ANS binding showed that GroEL and Lon compete for folding intermediate of DHFR and their
relative concentrations determines the outcome. We developed a computational model to
analyze this competition, which lead us to the conclusion that our observations cannot be
reconciled with GroEL role as just caging device to protect DHFR mutants from aggregation
and proteolysis. Rather, it must play an active role in converting intermediate to folded
molecules.
Host: Prof. Nir Ben-Tal, NirB@tauex.tau.ac.il , Dept. of Biochemistry and
Molecular Biology, Faculty of Life Sciences, Tel Aviv University