Strain stiffening of Ndc80 complexes attached to microtubule plus ends

Mechanical coupling coordinates microtubule growth

“…To measure the effects of mechanical coupling, we recorded the growth of many pairs of microtubules coupled to a constant shared load ( F TOT = 8 pN) either through relatively soft springs, with κ = 1 pN·μm -1 , or through springs with five-fold greater stiffness, κ = 5 pN·μm -1 . These two stiffnesses span a range near the minimum elasticities estimated for mitotic chromosomes (Nicklas, 1983; Poirier et al, 2002), for pericentromeres (Chacon et al, 2014), and for elements of the kinetochore (Schwietert et al, 2022; Volkov et al, 2018). When coupled through soft springs (κ = 1 pN·μm -1 ), microtubule pairs tended to grow at different rates and their tips often became separated by more than a micrometer (Figure 3a, left, and Figure S3-1, left).…”

“…Using our dual-trap assay, we found that pairs of microtubule tips growing in vitro are kept in close proximity when sharing a single load through a coupling material of sufficient stiffness. Tip pairs in our experiments were kept within a fraction of a micrometer by couplers of only κ = 5 pN•µm -1 , a modest stiffness within the ranges reported for elements of the kinetochore (Schwietert et al, 2022;Volkov et al, 2018), pericentromeric chromatin (Chacon et al, 2014), and mitotic chromosomes (Nicklas, 1983;Poirier et al, 2002). Because the kinetochore and its underlying centromeric chromatin both appear to deform together to maintain close contacts with k-fiber plusends in vivo (O'Toole et al, 2020), the plus-ends presumably exert forces on each other through a composite material composed of both kinetochore and chromatin.…”

“…Nevertheless, the dual-trap approach can be used in the future (potentially with a cutting laser to sever the microtubule tips to induce shortening (Driver et al, 2017;Murray et al, 2022)) to study how mechanical coupling affects microtubule pairs in which one or both filaments are shortening, as well as switch rates between shortening and growth. Our dual-trap approach can also be adapted to examine mechanical coupling through materials with complex mechanical properties, such as viscoelasticity and strain-stiffening (Cojoc et al, 2016;Schwietert et al, 2022). In principle, the effect of mechanical coupling on pairs of processive motor proteins (Holzbaur & Goldman, 2010) or on pairs of dynamic actin filaments linked to a shared load (Garner & Theriot, 2022) can also be studied with the dual-trap approach.…”

Mechanical coupling coordinates microtubule growth

“…To measure the effects of mechanical coupling, we recorded the growth of many pairs of microtubules coupled to a constant shared load ( F TOT = 8 pN) either through relatively soft springs, with κ =1 pN·µm –1 , or through springs with five-fold greater stiffness, κ =5 pN·µm –1 . These two stiffnesses span a range near the minimum elasticities estimated for mitotic chromosomes ( Nicklas, 1983 ; Poirier et al, 2002 ), for pericentromeres ( Chacón et al, 2014 ), and for elements of the kinetochore ( Schwietert et al, 2022 ; Volkov et al, 2018 ). When coupled through soft springs ( κ =1 pN·µm –1 ), microtubule pairs tended to grow at different rates and their tips often became separated by more than a micrometer ( Figure 3a , left, and Figure 3—figure supplement 1 , left).…”

“…Thus far, we have simulated only purely elastic couplers, where both coupling springs are linear (Hookean) with stiffness, κ ( Figure 2b ). In the future, more complex coupling materials with viscoelasticity ( Cojoc et al, 2016 ) or strain-stiffening ( Schwietert et al, 2022 ; Volkov et al, 2018 ) could also be simulated.…”