Local nucleation of microtubule bundles through tubulin concentration into a condensed tau phase

Abstract

Non-centrosomal microtubule bundles play important roles in cellular organization and function. Although many diverse proteins are known that can bundle microtubules, biochemical mechanisms by which cells could locally control the nucleation and formation of microtubule bundles are understudied. Here, we demonstrate that concentration of tubulin into a condensed, liquid-like compartment composed of the unstructured neuronal protein tau is sufficient to nucleate microtubule bundles. We show that under conditions of macro-molecular crowding, tau forms liquid drops. Tubulin partitions into these drops, efficiently increasing tubulin concentration and driving the nucleation of microtubules. These growing microtubules form bundles enclosed in a liquid sheath of tau. Our data suggest that condensed compartments of microtubule bundling proteins could promote the local formation of microtubule bundles in neurons by acting as non-centrosomal microtubule nucleation centers, and that liquid-like tau encapsulation could provide both stability and plasticity to long axonal microtubule bundles.

Controlling compartmentalization by non-membrane-bound organelles

“…Condensates can wet surfaces [2], with the contact angle depending on the molecular interaction [38]. In cells, condensates appear to wet membrane surfaces [2] and cytoskeletal fibres [39].…”

Actin polymerization counteracts prewetting of N-WASP on supported lipid bilayers

“…Several examples of wetting phenomena have been observed in a biological context. Wetting of biological surfaces by protein condensates has been observed, for example, for transcription factors on DNA (28), for proteins on lipid membranes (40) and for proteins on microtubules (41). For the latter, TPX2 protein can wet microtubules and form a coat.…”

Optimization and application of Trim-Away for studying a liquid-like spindle domain in mammalian oocytes

“…Later, phase separation of BuGZ was also found to promote the activation of AURA, one of the kinases driving mitotic spindle assembly 59 . The neuronal protein Tau is another one whose condensates can co-condense tubulin and polymerize Tau-encapsulated microtubule bundles in vitro 60 . More recently, phase separation of microtubulestabilizing protein TPX2 was similarly found to co-condense tubulin, promoting microtubule polymerization from existing microtubules 61 .…”

“…), or miRFP(60) and subcloned them into pGEMHE(61) to obtain meGFP-AKAP450(62), AURA-meGFP(63), Bbs4-meGFP(64), meGFP-BBS14(65), TurboGFP-BUGZ (66), mCherry-CDK5RAP2 (67), CEP72-meGFP (68), mScarlet-CEP192 (69), CETN2-meGFP (70), meGFPcentrobin (71), mClover3-CHC17 (21), chTOG-mScarlet (72), mEmerald-CLIP170 (M. Davidson, Florida State University, FL), meGFP-CPAP (73), mClover3-GTSE1 (72), H2B-miRFP (4), meGFP-HOOK3 (74), meGFP-KIF2B (75), LRRC36-meGFP (68), LRRC45-meGFP (76), mClover3-MAP4-MTBD (77), 3×CyOFP-MAP4-MTBD (77), NEDD1-mCherry (78), meGFP-NEK2A (79), meGFP-P50 (80), meGFP-P150 (81), meGFP-PAR6a (82), PCM1-mClover3 (Source BioScience), PCM1-mPA-GFP (Source Bioscience), meGFP-Pericentrin (83), PRC1-3×mClover3 (72), meGFP-rootletin (84), meGFP-SAS6 (85), TACC3-mClover3 (72), TACC3-mPA-GFP (72), mPA-GFP-TPX2 (86), (bovine) TRIM21 (Thermo Fisher Scientific), (mouse) TRIM21 (87), and g-tubulin-meGFP (88). EB3-3×mCherry was subcloned from pEB3-mCherry (J. Ellenberg, EMBL, Heidelberg, Germany) into pGEMHE.…”