Actin growth profile in clathrin-mediated endocytosis

Abstract

Clathrin-mediated endocytosis in yeast is driven by a protein patch containing close to 100 different types of proteins. Among the proteins are 5000-10,000 copies of polymerized actin, and successful endocytosis requires growth of the actin network. Since it is not known exactly how actin network growth drives endocytosis, we calculate the spatial distribution of actin growth required to generate the force that drives the process. First, we establish the force distribution that must be supplied by actin growth, by combining membrane-bending profiles obtained via electron microscopy with established theories of membrane mechanics. Next we determine the profile of actin growth, using a continuum mechanics approach and an iterative procedure starting with an actin growth profile obtained from a linear analysis. The profile has fairly constant growth outside a central hole of radius 45-50 nm, but very little growth in this hole. This growth profile can reproduce the required forces if the actin shear modulus exceeds 80 kPa, and the growing filaments can exert very large polymerization forces. The growth profile prediction could be tested via electron-microscopy or superresolution experiments in which the turgor pressure is suddenly turned off.

Molecular mechanisms of force production in clathrin‐mediated endocytosis

“…It is generally assumed that the membrane reaches steady state at each time step, because its relaxation times are on the millisecond timescale. Simulations by Carlsson and colleagues, which treated the actin meshwork as a continuous elastic material, were able to produce a pulling stress as large as 500 kPa on the CCP [25]. However, the result of this study implied that each growing actin filament must produce forces of 15 pN, which is significantly larger than the stalling force of individual filaments, as discussed above.…”

Single-molecule turnover dynamics of actin and membrane coat proteins in clathrin-mediated endocytosis

“…Theoretical calculations estimate that the amount of force needed to counteract turgor pressure and invaginate the membrane during CME in yeast is in the range of 1000-3000 pN (Carlsson and Bayly, 2014; Dmitrieff and Nedelec, 2015; Lacy et al, 2018; Tweten et al, 2017; Wang and Carlsson, 2017). The endocytic actin meshwork is composed of 100-200 short, Arp2/3-branched actin filaments that are capped shortly after nucleation; quantitative modeling indicates fewer than ten filament ends are polymerizing at any given time (Berro and Pollard, 2014; Sirotkin et al, 2010), each capable of producing around 1 pN of force (Footer et al, 2007; Kovar and Pollard, 2004).…”

A master equation approach to actin polymerization applied to endocytosis in yeast

“…Although the absence of an actin cortex in yeast precludes cortical tension, tension and compression are both present inside the endocytic actin patch. Despite the semiflexible nature of the actin filaments, assuming infinite rigidity may be reasonable on the basis of our previous estimate [ 50 ] of the Young’s modulus of the actin patch in the range of 140 kPa to 500 kPa. These high values result from the large density of crosslinkers in the patch.…”