Beyond the GTP‐cap: Elucidating the molecular mechanisms of microtubule catastrophe

Abstract

Almost 40 years since the discovery of microtubule dynamic instability, the molecular mechanisms underlying microtubule dynamics remain an area of intense research interest. The “standard model” of microtubule dynamics implicates a “cap” of GTP‐bound tubulin dimers at the growing microtubule end as the main determinant of microtubule stability. Loss of the GTP‐cap leads to microtubule “catastrophe,” a switch‐like transition from microtubule growth to shrinkage. However, recent studies, using biochemical in vitro reconstitution, cryo‐EM, and computational modeling approaches, challenge the simple GTP‐cap model. Instead, a new perspective on the mechanisms of microtubule dynamics is emerging. In this view, highly dynamic transitions between different structural conformations of the growing microtubule end – which may or may not be directly linked to the nucleotide content at the microtubule end – ultimately drive microtubule catastrophe.

Interface-acting nucleotide controls polymerization dynamics at microtubule plus- and minus-ends

“…Indeed, the tendency for plus-ended growth to ‘stutter’ (Mahserejian et al, 2022) and fluctuate (Cleary et al, 2022) might be explained by exposed GDP-tubulin; exposed GDP-tubulin may also contribute to the higher frequency of catastrophe at the plus-end (Strothman et al, 2019; Walker et al, 1988). Our work supports an emerging view of the growing microtubule end as a ‘mosaic’ of nucleotide states rather than a uniform assembly of GTP-tubulin (Brouhard & Sept, 2012; Brouhard & Rice, 2018; Cross, 2019; Duellberg et al, 2016; Farmer et al, 2021; Farmer & Zanic, 2023; Gudimchuk & McIntosh, 2021; Howard & Hyman, 2009; Margolin et al, 2012; Maurer et al, 2012; Roostalu et al, 2020; Roth et al, 2018). By allowing for the possibility of multiple nucleotide states on the microtubule end, our work also resonates with recent studies of the microtubule regulatory factor CLASP (Lawrence et al, 2022; Luo et al, 2023), which regulates microtubule plus-ends differently depending on the nucleotide state of the terminal subunit at the protofilament plus-end.…”

“…Our results point to a more nuanced view of the GTP cap model, which posits that growing microtubule ends are protected against depolymerization by a ‘cap’ of GTP-tubulin ((Mitchison & Kirschner, 1984), reviewed in (Gudimchuk & McIntosh, 2021)). Early views of the GTP cap did not anticipate the influence of GDP-tubulin on growing plus-ends, but there is now increasing evidence (Carlier & Pantaloni, 1978; Farmer & Zanic, 2023; Hamel et al, 1986; Margolin et al, 2012; Maurer et al, 2012; Roth et al, 2018; Valiron et al, 2010) that the cap is not ‘all or nothing’, and that GDP-tubulin can modulate microtubule growth without always initiating a catastrophe. Indeed, the tendency for plus-ended growth to ‘stutter’ (Mahserejian et al, 2022) and fluctuate (Cleary et al, 2022) might be explained by exposed GDP-tubulin; exposed GDP-tubulin may also contribute to the higher frequency of catastrophe at the plus-end (Strothman et al, 2019; Walker et al, 1988).…”

CLASPs stabilize the intermediate state between microtubule growth and catastrophe

“…Our finding that CLASP promotes slow microtubule depolymerization in the presence of GTP, GTPγS, and GDP, but not GMPCPP, suggests that the intermediate state requires a post-GTP hydrolysis state. Indeed, it has been proposed that the transient exposure of terminal GDP-tubulin dimers during growth serves as a precursor to catastrophe (Bowne-Anderson et al 2013, Farmer and Zanic 2022). Exposure of GDP-tubulin at growing microtubule ends has recently been linked to an increase in microtubule growth fluctuations and slowed microtubule growth (Cleary et al 2022).…”

Chemical-induced changes in the optical properties of aromatic amino acid lattices in protein polymers

“…While the 1JFF structure provides tubulin in a straight protofilament conformation, as in microtubules, the 4O2B structure provides a bent conformation like that observed following guanosine triphosphate hydrolysis by tubulin. This suggests that alterations of tubulin conformation, such as following hydrolysis of guanosine triphosphate or in microtubule catastrophe [31] will present with altered energy migration patterns.…”