Tsuji, C. | bioRxiv Science

bioRxiv2023

CryoET reveals actin filaments within platelet microtubules

Crosstalk between the actin and microtubule cytoskeletons is essential for many cellular processes. Recent studies have shown that microtubules and F-actin can assemble to form a composite structure where F-actin occupies the microtubule lumen. Whether these cytoskeletal hybrids exist in physiological settings and how they are formed is unclear. Here, we show that the short-crossover Class I actin filament previously identified inside microtubules in human HAP1 cells is cofilin-bound F-actin. Lumenal F-actin can be reconstituted in vitro, but cofilin is not essential. Moreover, actin filaments with both cofilin-bound and canonical morphologies reside within human platelet microtubules under physiological conditions. We propose that stress placed upon the microtubule network during motor-driven microtubule looping and sliding may facilitate the incorporation of actin into microtubules.

cell biology↗

bioRxiv2020

Phospho-regulated auto-inhibition of Cnn controls microtubule nucleation during cell division

{gamma}-tubulin ring complexes ({gamma}-TuRCs) nucleate microtubules. They are recruited to centrosomes in dividing cells via binding to N-terminal CM1 domains within {gamma}-TuRC-tethering proteins, including Drosophila Cnn. Binding promotes microtubule nucleation and is restricted to centrosomes in dividing cells, but the mechanism regulating binding remains unknown. Here we identify an extreme N-terminal "CM1 auto-inhibition" (CAI) domain found specifically within the centrosomal isoform of Cnn (Cnn-C) that inhibits {gamma}-TuRC binding. Robust binding occurs after removal of the CAI domain or with the addition of phospho-mimetic mutations, suggesting that phosphorylation helps relieve inhibition. We show that regulation of Cnn binding to {gamma}-TuRCs is isoform-specific and that mis-regulation of binding can result in ectopic cytosolic microtubules and major defects during cell division. We also find that human CDK5RAP2 is auto-inhibited from binding {gamma}-TuRCs, suggesting conservation across species. Overall, our results shed light on how and why CM1 domain binding to {gamma}-TuRCs is regulated. Summary (for the online JCB table of contents and alerts)We show that auto-inhibition regulates the binding between microtubule nucleating complexes and proteins that tether them to sites of microtubule nucleation. Failure to properly regulate this binding can lead to ectopic cytosolic microtubule nucleation and major defects during cell division.

cell biology↗