Edward Egelman receives Competitive Renewal of Actin Studies from NIGMS

Project Summary:
Actin is one of the most abundant human proteins, but very little understanding exists about why actin
is so highly conserved. Over 350 million years of evolution there have been no amino acid changes in the
skeletal striated muscle isoform of actin. Similarly, there is no understanding of why most sequence differences
are between tissue-specific isoforms and not between species. The hypothesis that is being investigated in this
research is that many allosteric linkages exist within the actin filament, placing constraints and selective
pressure on all of the buried residues. These allosteric linkages are essential for the many functions of actin,
from muscle contraction to cell motility to forming the stereocilia responsible for hearing, and we postulate that
mutations of buried residues that break these allosteric linkages are responsible for a number of diseases.
Past productivity has been very strong in this project, but the proposed period represents a major
transformation. With our Titan Krios and Falcon II direct electron detector we can have near-atomic resolution
for the interactions within the actin filament and between actin and actin-binding proteins. The eight proposed
aims include studies of actin mutants, and many complexes of actin filaments with other proteins, including the
nucleocapsid (NC) domain of HIV-1. Some of these actin-binding proteins contain structurally conserved
domains, such as the Immunoglobulin (Ig) domains, and the hypothesis is that the presence of such domains
tells us only about the ancestry and evolution of these proteins and not about how these domains interact with
actin. Thus, different Ig domains may interact in a completely different manner with actin, or not interact at all,
as was shown for the CH domain within the eponymous protein calponin. The proposed studies will be done in
collaboration with other laboratories using biochemistry and mutagenesis, and the resulting work should have a
large impact on our understanding of how actin functions in a huge number of cellular processes and interacts
with many other proteins.