Scientists at Stony Brook University, led by Saikat Chowdhury, PhD, senior author and an assistant professor in the department of biochemistry and cell biology in the College of Arts and Sciences at Stony Brook University, say they have determined the structure of the protein Arp2/3 and describe it in a paper titled, “Cryo-EM reveals the transition of Arp2/3 complex from inactive to nucleation-competent state,” and published in Nature Structural and Molecular Biology.

More than twenty years ago, researchers discovered the Arp2/3 complex, an actin cytoskeketal nucleator that plays a crucial role in cell division, immune response, neurodevelopment, and other biological processes. But there has been no determined structure of the activated state of the complex until now, an achievement that may lay the foundation for uncovering its role in biology and in the development of disease, according to the Stony Brook team.

Chowdhury, and graduate student and first author Mohammed Shaaban determined the first near-atomic resolution structure of Arp2/3 in its active state by using cryo-electron microscopy. The structure shows the complex in its active form and bound to a signaling molecule. It also shows the nucleated actin filament, thus providing a structural snapshot of the global and local conformational changes in the Arp2/3 complex that help grow new actin filament in cells.

“Arp2/3 complex, a crucial actin filament nucleator, undergoes structural rearrangements during activation by nucleation-promoting factors (NPFs). However, the conformational pathway leading to the nucleation-competent state is unclear due to lack of high-resolution structures of the activated state. Here we report a ~3.9 Å resolution cryo-EM structure of activated Schizosaccharomyces pombe Arp2/3 complex bound to the S. pombe NPF Dip1 and attached to the end of the nucleated actin filament,” the investigators wrote.

The structure reveals global and local conformational changes that allow the two actin-related proteins in Arp2/3 complex to mimic a filamentous actin dimer and template nucleation. Activation occurs through a clamp-twisting mechanism, in which Dip1 forces two core subunits in Arp2/3 complex to pivot around one another, shifting half of the complex into a new activated position. By showing how Dip1 stimulates activation, the structure reveals how NPFs can activate Arp2/3 complex in diverse cellular processes.”

“Obtaining the macromolecular structure of activated Arp2/3 complex has been a long-standing goal for scientists,” said Chowdhury.  “Our structure reveals a level of molecular details which show the individual components of the complex and how they are positioned relative to each other in the active state.”

Having a structure of Arp2/3 in its active state will help drive more detailed research of the complex. Chowdhury explained that this is extremely important because when Arp2/3 is deregulated in the biological state, it is associated with cancer metastasis, neurodegeneration, bacterial and viral infections, and wound healing problems.

“So not only does this structure enable us to fill a knowledge gap in the actin cell biology field, it potentially helps to build our understanding of the underlying causes of a number of diseases with the ultimate goal of developing new therapeutics,” emphasized Chowdhury.

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