Actin assembly produces sufficient forces for endocytosis in yeast

Abstract

We formulated a spatially resolved model to estimate forces exerted by a polymerizing actin meshwork on an invagination of the plasma membrane during endocytosis in yeast cells. The model, which approximates the actin meshwork as a visco-active gel exerting forces on a rigid spherocylinder representing the endocytic invagination, is tightly constrained by experimental data. Simulations of the model produce forces that can overcome resistance of turgor pressure in yeast cells. Strong forces emerge due to the high density of polymerized actin in the vicinity of the invagination and because of entanglement of the meshwork due to its dendritic structure and cross-linking. The model predicts forces orthogonal to the invagination that are consistent with formation of a flask shape, which would diminish the net force due to turgor pressure. Simulations of the model with either two rings of nucleation-promoting factors (NPFs) as in fission yeast or a single ring of NPFs as in budding yeast produce enough force to elongate the invagination against the turgor pressure.

Direct comparison of clathrin-mediated endocytosis in budding and fission yeast reveals conserved and evolvable features

“…proposed for budding yeast (Figure 1-figure supplement1) is sufficient for fission yeast CME, 30 consistent with a recent models of force production by actin assembly (Nickaeen et al, 2019). 31…”

“…Quantitation of protein numbers at endocytic sites was a critical, pioneering 23 development in fission yeast studies (Sirotkin et al, 2010). Comparatively lower numbers of 24 homologous proteins were reported present at budding yeast endocytic sites (Picco et al, 2015), 25 and in some cases discrepancies were reported even for the same protein in the same yeast species comparisons. Furthermore, the dynamics of some of the proteins including WASp, one of the 29 main nucleation promoting factors (NPFs) for the Arp2/3 complex, were reported to be distinctly 30 different in budding and fission yeast (Kaksonen et al, 2003;Sirotkin et al, 2010).…”

FisB relies on homo-oligomerization and lipid-binding to catalyze membrane fission in bacteria

“…Both fission machineries bind acidic lipids, assemble into oligomers, and use hydrolysis of a nucleoside triphosphate (GTP or ATP) to achieve membrane fission. However, membrane fission can also be achieved by friction 16 , stress accumulated at a boundary between lipid domains 17 , forces generated by the acto-myosin network [18][19][20][21] or protein crowding 22 . By contrast, very little is known about membrane fission in bacteria, even though they rely on membrane fission for every division cycle.…”

Integrating protein copy numbers with interaction networks to quantify stoichiometry in mammalian endocytosis

“…However, this is currently intractable, given the sheer number of components and interactions, coupling to cytoskeleton 10,29–33 , membrane bending and vesicle fission 34–36 , and connection to downstream steps in vesicle trafficking. Modeling efforts have nonetheless been critical in helping quantify requirements for clathrin-cage assembly 23,37–42 , the role of the cytoskeleton 33,43,44 , the impact of dimensionality reduction on assembly 23,24 , and the mechanical coupling to the membrane 45–47 . Although the sequence of events in CME have been broadly defined 10 (including initiation by localization of adaptors, recruitment and assembly of the clathrin lattice, and concurrent bending of the membrane), computational approaches such as ours help to predict how perturbations to single components or subsets of components will propagate throughout the network.…”