High interaction valency ensures cohesion and persistence of a microtubule +TIP body at the plus-end of a single specialized microtubule in yeast

Abstract

Microtubule plus-end tracking proteins (+TIPs) control microtubule specialization and are as such essential notably during eukaryotic cell division. Here, we investigated interactions and functions of the budding yeast Kar9 network consisting of the core +TIPs components Kar9 (functional homologue of APC, MACF, and SLAIN), Bim1 (orthologue of EB1), and Bik1 (orthologue of CLIP-170). Our data indicate that a redundant, multivalent web of interactions links the three +TIPs together to form a "Kar9 body" at the tip of a single cytoplasmic microtubule. They further suggest that this body is a liquid-like condensate, allowing it to persist on both growing and shrinking microtubule tips, and functions as a mechanical coupling device between microtubules and actin cables during mitosis. Our study underlines the power of dissecting the web of low-affinity interactions driving liquid-liquid phase separation of proteins in order to demonstrate the importance and establish the functional roles of condensation processes in vivo.

Phase separation of +TIP-networks regulates microtubule dynamics

“…Further studies will be necessary to investigate whether additional +TIPs contribute to the formation and regulation of +TIP-droplets. Our work here and recent studies demonstrate that +TIP networks can behave like liquid condensates (Wu et al, 2021;Maan et al, 2021;Meier et al, 2021;Song et al, 2021). This work adds to the growing list of microtubule-related processes that are driven by LLPS and provides an exciting new paradigm for how cells can spatiotemporally control microtubule dynamics through local tubulin concentration (Zhang et al, 2015;Woodruff et al, 2017;Hernández-Vega et al, 2017;King and Petry, 2020;Jiang et al, 2021;Maan et al, 2021;Meier et al, 2021;Song et al, 2021).…”

“…The role of different mammalian EB family members in regulating LLPS will be an interesting direction for future studies. The ability of these +TIP-networks to phase separate depends on intrinsically disordered regions (Maan et al, 2021;Song et al, 2021) and multivalent interaction modules (Figure 2; Meier et al, 2021), consistent with the observation that these features are highly evolutionarily conserved across +TIPs (Wu et al, 2021). Further studies will be necessary to investigate whether additional +TIPs contribute to the formation and regulation of +TIP-droplets.…”

Phase separation of +TIP-networks regulates microtubule dynamics

“…Our work here and recent studies demonstrate that +TIP networks can behave like liquid condensates (Wu et al, 2021;Maan et al, 2021;Meier et al, 2021;Song et al, 2021). This work adds to the growing list of microtubule-related processes that are driven by LLPS and provides an exciting new paradigm for how cells can spatiotemporally control microtubule dynamics through local tubulin concentration (Zhang et al, 2015;Woodruff et al, 2017;Hernández-Vega et al, 2017;King and Petry, 2020;Jiang et al, 2021;Maan et al, 2021;Meier et al, 2021;Song et al, 2021). Interrogating the mechanical properties and composition of +TIP-droplets, as well as studying their regulation throughout the cell cycle, will be exciting avenues for future research.…”

“…Is phase separation a common feature of +TIPs? Studies performed in parallel to our work show that this phenomenon is conserved across evolution: +TIPs in budding yeast, fission yeast, and higher eukaryotes have recently been demonstrated to undergo phase separation (Maan et al, 2021;Meier et al, 2021;Song et al, 2021, Jijumi et al, 2022. Intriguingly, in line with our results, the yeast studies confirmed that the CLIP-170 homolog played a key role in the phase separation process, whereas LLPS potency of EB homologs varied between organisms.…”

“…The role of different mammalian EB family members in regulating LLPS will be an interesting direction for future studies. The ability of +TIP-networks to phase separate depends on intrinsically disordered regions (Maan et al, 2021;Song et al, 2021) and multivalent interaction modules (Meier et al, 2021), consistent with the observation that these features are highly evolutionarily conserved across +TIPs (Wu et al, 2021). Further studies will be necessary to investigate whether additional +TIPs contribute to the formation and regulation of +TIP-droplets.…”