Researchers discover new complexity in actin dynamics

Within a single human cell, a fascinating world of complexity unravels, with around 100,000 diverse proteins bustling with activity. Among them, actin stands out as one of the most crucial and abundant proteins. It takes center stage in forming filaments that compose the cell’s framework, providing shape and structure. These actin filaments, as they elongate, perform the role of muscles, exerting force against the inner membrane to propel the cell forward.

But actin’s performance doesn’t happen in isolation. Three other proteins act as conductors, directing the activities of actin. One assembles individual actin molecules into filaments, another regulates their growth, and a third dismantles filaments when needed.

Intriguingly, scientists at Emory University have unveiled a more intricate and nuanced interplay between these three proteins, revealing how they occasionally transition from solo or paired performances to form a trio. This collaboration allows them to finely control the behavior of actin filaments.

The published findings in Nature Communications have opened a new window into the dynamic world of cellular movement, a process critical for various functions, from stem-cell differentiation and wound healing to the onset of diseases like cancer.

Emory’s Assistant Professor of Physics and Cell Biology, Shashank Shekhar, the study’s senior author, explains, “We found that while these three proteins do one thing when working on their own, they do a completely different thing when the other two proteins join them. It gets really complex, very fast.”

Co-first author of the study, Heidi Ulrichs, an Emory Ph.D. candidate in biochemistry, cell, and developmental biology, adds, “No one had looked at all of these proteins interacting at once on actin. Our paper is the first report of all three of them occupying the same barbed end of an actin filament.”

This groundbreaking research has paved the way for a deeper understanding of cellular behavior, offering potential insights into various biological processes, from the intricacies of stem cells to wound healing and disease progression, such as cancer.

Building on previous research

The intricate actions of individual proteins on actin have been extensively studied and well-characterized. Among them, formin, a polymerase protein, plays a pivotal role in elongating actin filaments. It strategically situates itself at the end of an actin filament, adeptly binding to free-floating actin molecules and stacking them up sequentially to facilitate continuous growth.

On the other hand, twinfilin, a depolymerase protein, exerts its influence on actin in a unique manner. It functions akin to a lint roller, adhering to the end of a filament and progressively peeling away individual actin molecules, leading to the disassembly of the filament. Twinfilin can repeat this process multiple times, ultimately dismantling the entire actin structure.

Moreover, there are capper proteins that act as regulators, halting the elongation and disassembly of actin filaments. These cappers bind themselves to the filament’s end, forming a protective cover akin to a hat, effectively blocking the activity of other proteins involved.

The knowledge of these individual protein actions on actin has been acquired through meticulous isolation and focused study of each protein’s role. Recent research has further unveiled intriguing simultaneous interactions, particularly between twinfilin and capping proteins, adding layers of complexity to our understanding of actin dynamics. As scientists continue to unravel these intricate interactions, a more comprehensive and nuanced picture of cellular movement emerges.

A new approach using advanced technology

In their latest study, the researchers aimed to investigate whether formin, twinfilin, and the capping protein could simultaneously act on actin, despite the limited space available at the tiny, five-nanometer-wide end of an actin filament.

To explore this, the Shekhar Lab utilized a specialized technique called microfluidics-assisted total internal reflection fluorescence microscopy (mf-TIRF) to observe how the actin cytoskeleton remodels itself. This approach involves tagging individual protein molecules with fluorescent orbs, allowing for precise observation under a microscope.

The experiments involved introducing actin into the microfluidic system and then adding the other proteins one by one. The researchers used four different fluorescent colors to distinguish actin, formin, twinfilin, and the capping protein.

The results were astonishing. When twinfilin, known for breaking apart actin filaments, was added alongside formin and the capping protein, it unexpectedly accelerated the filament elongation process. This counterintuitive discovery surprised the researchers and added a fascinating layer of complexity to their findings.

Interestingly, twinfilin couldn’t act on the end of the actin filament when present alone. However, with the capping protein also in the mix, all three proteins could collaboratively function on the small surface of the filament.

The researchers likened the combined effects of the three proteins to a knob that allows for precise control over the filament formation speed, establishing a new paradigm for their coordinated action.

Understanding the intricate dynamics of how these three proteins interact with actin holds the key to unraveling the complex mechanisms underlying normal cellular function and what goes awry in disease conditions.

The step-by-step accumulation of knowledge from studies like this contributes to our growing understanding of the inner workings of cells and offers potential insights into various cellular processes. As Heidi Ulrichs aptly puts it, “We’re building up knowledge, step by step, study by study, on the dynamics of what’s happening inside of a cell.”

Source: Emory University

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