UCLA researchers have found that modification of actin leads to dramatic changes in actin filament stability.
In a study published in December in Nature Communications, researchers led by Professors Emil Reisler (UCLA Department of Chemistry and Biochemistry) and Hong Zhou (UCLA Department of Microbiology, Immunology and Molecular Genetics) demonstrate that modification of actin’s methionine residues leads to rapid depolymerization of actin filaments. Via structural studies, the researchers discover that oxidation of two of actin’s methionine residues by Mical family enzymes leads to unstable contacts between actin subunits and thus much faster rates of filaments depolymerization.
Actin plays an important role in cell structure and function. While many proteins that bind to and regulate the polymerization of actin monomers or depolymerization of actin filaments have been identified, fewer studies identify regulation of filament dynamics via post-translational modifications of actin itself.
Mical oxidized actin filaments (red) disassemble much faster than unoxidized F-actin (green) in single filament TIRF assays. Disassembly catastrophes are indicated with blue arrowheads. (Figure courtesy of Reisler group)
Using TIRF microscopy to look at single actin filaments, the researchers first discovered that Mical-oxidized actin (Mox-actin) polymerized about three times slower and depolymerized about fifteen times faster than unmodified actin. The researchers noted that Mox-actin is more stable when it is “young” (with ATP or ADP-Pi -bound to it) ) and that “old” Mox-actin (with ADP bound to it) is responsible for the observed rapid disassembly.
Via cryo-EM, the researchers obtained structures of Mox-actin filaments to 3.9 Angstrom resolution and discovered that Mox-actin adopts two different conformations: One conformation closely resembled the structure of normal actin filaments, but in the other conformation, the researchers observed structural changes in actin monomers near where they join to form a polymer.
The researchers suggest this change forces Mox-actin filaments to adopt an unstable conformation, explaining their tendency to quickly and suddenly depolymerize.
Dr. Elena Grintsevich (pictured right) is a researcher in the Reisler lab and the first author on the study. Also on the study were Dr. Peng Ge from the California NanoSystems Institute (CNSI), Dr. Michael Sawaya from the Molecular Biology Institute (MBI) and the UCLA-DOE institute, and researchers from the lab of Professor Jonathan Terman at University of Texas (UT) Southwestern Medical Center in Dallas.
Many thanks to Joseph Ong for writing this article.