Soluble tubulin is significantly enriched at mitotic centrosomes

Abstract

During mitosis, the centrosome expands its capacity to nucleate microtubules. Understanding the mechanisms of centrosomal microtubule nucleation is, however, constrained by a lack of knowledge of the amount of soluble and polymeric tubulin at mitotic centrosomes. Here we combined light microscopy and serial-section electron tomography to measure the amount of dimeric and polymeric tubulin at mitotic centrosomes in early C. elegans embryos. We show that a C. elegans one-cell stage centrosome at metaphase contains 10,000 microtubules with a total polymer concentration of 230 µM. Centrosomes concentrate soluble α/β tubulin by about 10-fold over the cytoplasm, reaching peak values of 470 µM, giving a combined total monomer and polymer tubulin concentration at centrosomes of up to 660 µM. These findings support in vitro data suggesting that microtubule nucleation in C. elegans centrosomes is driven in part by concentrating soluble tubulin.

Drops and fibers — how biomolecular condensates and cytoskeletal filaments influence each other

“…Supporting evidence comes from work in vivo showing that soluble tubulin concentrates ∼10 fold with peak values of 470 mM in C. elegans centrosomes over the surrounding cytosolic concentration (Figure 1c). This is about an order of magnitude higher than the concentration required for spontaneous nucleation of tubulin in a test tube, providing strong evidence for concentration-enhanced microtubule nucleation at centrosomes [58]. In acentrosomal oocytes, the regulatory kinase aurora A (AURA), its substrate TACC3 and the clathrin heavy chain CHC17 (both binding microtubules) have been identified to form a liquid-like meiotic spindle domain [59].…”

Polo-like kinase 1 independently controls microtubule-nucleating capacity and size of the centrosome

“…As PCM assembly is unaffected in Cluster II mut embryos (Fig. 2C), a second possibility is that the remaining spindle microtubules are nucleated by g-tubulin-independent mechanisms involving other PCM-localized factors and/or the ability of centrosomes to locally concentrate soluble tubulin (Baumgart et al, 2019;Woodruff et al, 2017). A serendipitous observation leads us to favor the first possibility.…”

DrosophiladTBCE recruits tubulin around chromatin to promote mitotic spindle assembly

“…Moreover, it has been shown that tubulin enrichment at the centrosome by MAPs may be an essential mechanism to promote nucleation of MTs 11,12 . It will thus be interesting to investigate whether local increase in tubulin concentration by tubulin-interacting proteins is a general and conserved mechanism involved in the spatiotemporal regulation of MTs.…”

“…Indeed, the Chromosome Passenger Complex (CPC) also contributes to MT nucleation and can compensate the absence of Ran 9,10. Interestingly, even if in vitro MT nucleation is known to be dependent upon tubulin concentration, local accumulation of free tubulin has only recently started to be considered as a key element that could significantly contribute to spindle assembly 11,12 . Surprisingly, in C.elegans and D.…”