Biology subjects

Mladenov, M.

Publications and source records attributed to Mladenov, M..

4 recordsLinked to original sources

bioRxiv2023

CK-666 and CK-869 differentially inhibit Arp2/3 iso-complexes

The inhibitors, CK-666 and CK-869, are widely used to probe the function of actin nucleation by the Arp2/3 complex in vitro and in cells. However, in mammals, the Arp2/3 complex consists of 8 iso-complexes, as three of its subunits (Arp3, ArpC1, ArpC5) are encoded by two different genes. Here, we used recombinant Arp2/3 with defined composition to assess the activity of CK-666 and CK-869 against iso-complexes. We demonstrate that both inhibitors prevent linear actin filament formation when ArpC1A- or ArpC1B-containing complexes are activated by SPIN90. In contrast, inhibition of actin branching depends on iso-complex composition. Both drugs prevent actin branch formation by complexes containing ArpC1A, but only CK-869 can inhibit ArpC1B-containing complexes. Consistent with this, in bone marrow-derived macrophages which express low levels of ArpC1A, CK-869 but not CK-666, impacted phagocytosis and cell migration. CK-869 is also only able to inhibit Arp3-but not Arp3B-containing iso-complexes. Our findings have important implications for the interpretation of results using CK-666 and CK-869, given that the relative expression levels of ArpC1 and Arp3 isoforms in cells and tissues remains largely unknown.

biochemistry↗

bioRxiv2023

Regeneration of actin filament branches from the same Arp2/3 complex

Branched actin filaments are found in many key cellular structures. Branches are nucleated by the Arp2/3 complex activated by nucleation-promoting factor (NPF) proteins and bound to the side of pre-existing mother filaments. Over time, branches dissociate from their mother filament, leading to network reorganization and turnover, but this mechanism is less understood. Here, using microfluidics and purified proteins, we examined the dissociation of individual branches under controlled biochemical and mechanical conditions. We observe that Arp2/3 remains bound to the mother filament after most debranching events, even when accelerated by force. Unexpectedly, this mother-remaining Arp2/3 readily nucleates a new actin filament branch, without being activated anew by an NPF: it simply needs to exchange its nucleotide and bind an actin monomer. The protein GMF, which accelerates debranching, prevents branch re-nucleation. Our results suggest that actin filament re-nucleation can provide a self-repair mechanism, helping branched networks to sustain mechanical stress in cells over extended periods of time.

biophysics↗

bioRxiv2023

Cortactin stabilizes actin branches by bridging activated Arp2/3 to its nucleated actin filament

Regulation of the assembly and turnover of branched actin filament networks nucleated by the Arp2/3 complex is essential during many cellular processes including cell migration and membrane trafficking. Cortactin plays a key role in stabilizing actin filament branches by interacting with the Arp2/3 complex and actin filaments via its N-terminal Acidic domain (NtA) and 6.5 central unstructured 37 amino acid repeats, respectively 1, but the mechanism of this is unclear. We determined the structure of cortactin-stabilized Arp2/3 actin branches using cryo-electron microscopy. We find that cortactin interacts with the new daughter filament nucleated by the Arp2/3 complex at the branch site rather than the initial mother actin filament. Cortactin preferentially binds activated Arp3 in contrast to other nucleation promoting factors (NPFs) 2,3. Cortactin also stabilizes the F-actin-like interface of activated Arp3 with the first actin subunit of the new filament, and its central repeats extend along successive daughter filament subunits. Cortactin binding to Arp3 is incompatible with NPF interaction and its preference for activated Arp3 explains why it is retained at the actin branch. Our data have uncovered why cortactin displaces NPFs, while at the same time promoting synergy to regulate branched actin network dynamics.

biophysics↗

bioRxiv2023

Biomimetic actin cortices shape cell-sized lipid vesicles

Animal cells are shaped by a thin layer of actin filaments underneath the plasma membrane known as the actin cortex. This cortex stiffens the cell surface and thus opposes cellular deformation, yet also actively generates membrane protrusions by exerting polymerization forces. It is unclear how the interplay between these two opposing mechanical functions plays out to shape the cell surface. To answer this question, we reconstitute biomimetic actin cortices nucleated by the Arp2/3 complex inside cell-sized lipid vesicles. We show that thin Arp2/3-nucleated actin cortices strongly deform and rigidify the shapes of giant unilamellar vesicles and impart a shape memory on time scales that exceeds the time of actin turnover. In addition, actin cortices can produce finger-like membrane protrusions, showing that Arp2/3-mediated actin polymerization forces alone are sufficient to initiate protrusions in the absence of actin bundling or membrane curving proteins. Combining mathematical modeling and our experimental results reveals that the concentration of actin nucleating proteins, rather than actin polymerization speed, is crucial for protrusion formation. This is because locally concentrated actin polymerization forces can drive a positive feedback loop between recruitment of actin and its nucleators to drive membrane deformation. Our work paints a picture where the actin cortex can either drive or inhibit deformations depending on the local distribution of nucleators. Significance StatementThe cells in our body must actively change shape in order to migrate, grow and divide, but they also need to maintain their shape to withstand external forces during tissue development. Cellular shape control results from an interplay between the plasma membrane and its underlying cortex, a shell composed of crosslinked actin filaments. Using cell-free reconstitution and mathematical modelling, we show that minimal biomimetic actin cortices can mechanically rigidify lipid vesicles while at the same time driving membrane protrusion formation. Our observations suggest that the spatial distribution of actin nucleation determines whether the actin cortex drives or inhibits membrane deformations.

biophysics↗