Microtubule-severing enzymes

Structures and mechanisms of AAA+ protein complexes in DNA processing

AAA+ proteins are a large family of ATPases involved in a myriad of cellular activities. Recent advances in AAA+ proteins, especially cryoEM structures of these proteins in complex with their substrates, have provided key insights into how they function. Here we review recent progress in structural studies and mechanistic understanding of AAA+ proteins involved in DNA processing, including gene transcription, DNA replication, repair/recombination and transposition. Using a few selected examples, we show how AAA+ proteins act on both DNA and protein peptides, which are often enclosed in the pores of AAA+ hexamers. We propose that using AAA+ proteins to translocate a peptide to partially unfold a substrate is an effective strategy in disassembling an assembled complex. Further, several studies show that although they often act as asymmetric hexamers in their active form, AAA+ proteins adopt a range of oligomers for their functions.

Katanin regulatory subunit B1 (KATNB1) regulates BTB dynamics through changes in cytoskeletal organization

In this study, we have explored the role of the KATNB1 gene, a microtubule-severing protein, in the seminiferous epithelium of the rat testis. Our data have shown that KATNB1 expressed in rat brain, testes, and Sertoli cells. KATNB1 was found to co-localize with α-tubulin showing a unique stage-specific distribution across the seminiferous epithelium. Knockdown of KATNB1 by RNAi led to significant disruption of the tight junction (TJ) permeability barrier function in primary Sertoli cells cultured in vitro with an established functional TJ-barrier, as well as perturbations in the microtubule and actin cytoskeleton organization. The disruption in these cytoskeletal structures, in turn, led to improper distribution of TJ and basal ES proteins essential for maintaining the Sertoli TJ function. More importantly, overexpression of KATNB1 in the testis in vivo was found to block cadmium-induced blood-testis barrier (BTB) disruption and testis injury. KATNB1 exerted its promoting effects on BTB and spermatogenesis through corrective spatiotemporal expression of actin- and microtubule-based regulatory proteins by maintaining the proper organization of cytoskeletons in the testis, illustrating its plausible therapeutic implication. In summary, Katanin regulatory subunit B1 (KATNB1) plays a crucial role in BTB and spermatogenesis through its effects on the actin- and microtubule-based cytoskeletons in Sertoli cells and testis, providing important insights into male reproductive biology.

Microtubule-binding domains in Katanin p80 subunit are essential for severing activity in C. elegans

Microtubule-severing enzymes (MSEs), such as Katanin, Spastin, and Fidgetin play essential roles in cell division and neurogenesis. They damage the microtubule (MT) lattice, which can either destroy or amplify the MT cytoskeleton, depending on the cellular context. However, little is known about how they interact with their substrates. We have identified the microtubule-binding domains (MTBD) required for Katanin function in C. elegans. Katanin is a heterohexamer of dimers containing a catalytic subunit p60 and a regulatory subunit p80, both of which are essential for female meiotic spindle assembly. Here, we report that p80-like(MEI-2) dictates Katanin binding to MTs via two MTBDs composed of basic patches. Substituting these patches reduces Katanin binding to MTs, compromising its function in female meiotic-spindle assembly. Structural alignments of p80-like(MEI-2) with p80s from different species revealed that the MTBDs are evolutionarily conserved, even if the specific amino acids involved vary. Our findings highlight the critical importance of the regulatory subunit (p80) in providing MT binding to the Katanin complex.

Ultrasensitivity of microtubule severing due to damage repair

Microtubule-based cytoskeletal structures aid in cell motility, cell polarization, and intracellular transport. These functions require a coordinated effort of regulatory proteins which interact with microtubule cytoskeleton distinctively. In-vitro experiments have shown that free tubulin can repair nanoscale damages of microtubules created by severing proteins. Based on this observation, we theoretically analyze microtubule severing as a competition between the processes of damage spreading and tubulin-induced repair. We demonstrate that this model is in quantitative agreement with in-vitro experiments and predict the existence of a critical tubulin concentration above which severing becomes rare, fast, and sensitive to concentration of free tubulin. We show that this sensitivity leads to a dramatic increase in the dynamic range of steady-state microtubule lengths when the free tubulin concentration is varied, and microtubule lengths are controlled by severing. Our work demonstrates how synergy between tubulin and microtubule-associated proteins can bring about specific dynamical properties of microtubules.

14-3-3 protein augments the protein stability of phosphorylated spastin and promotes the recovery of spinal cord injury through its agonist intervention

Axon regeneration is abortive in the central nervous system following injury. Orchestrating microtubule dynamics has emerged as a promising approach to improve axonal regeneration. The microtubule severing enzyme spastin is essential for axonal development and regeneration through remodeling of microtubule arrangement. To date, however, little is known regarding the mechanisms underlying spastin action in neural regeneration after spinal cord injury. Here, we use glutathione transferase pulldown and immunoprecipitation assays to demonstrate that 14-3-3 interacts with spastin, both in vivo and in vitro, via spastin Ser233 phosphorylation. Moreover, we show that 14-3-3 protects spastin from degradation by inhibiting the ubiquitination pathway and upregulates the spastin-dependent severing ability. Furthermore, the 14-3-3 agonist Fusicoccin (FC-A) promotes neurite outgrowth and regeneration in vitro which needs spastin activation. Western blot and immunofluorescence results revealed that 14-3-3 protein is upregulated in the neuronal compartment after spinal cord injury in vivo. In addition, administration of FC-A not only promotes locomotor recovery, but also nerve regeneration following spinal cord injury in both contusion and lateral hemisection models; however, the application of spastin inhibitor spastazoline successfully reverses these phenomena. Taken together, these results indicate that 14-3-3 is a molecular switch that regulates spastin protein levels, and the small molecule 14-3-3 agonist FC-A effectively mediates the recovery of spinal cord injury in mice which requires spastin participation.

The katanin A-subunits KATNA1 and KATNAL1 act co-operatively in mammalian meiosis and spermiogenesis to achieve male fertility

Katanins, a class of microtubule-severing enzymes, are potent M-phase regulators in oocytes and somatic cells. How the complex and evolutionarily crucial, male mammalian meiotic spindle is sculpted remains unknown. Here, using multiple single and double gene knockout mice, we reveal that the canonical katanin A-subunit KATNA1 and its close paralogue KATNAL1 together execute multiple aspects of meiosis. We show KATNA1 and KATNAL1 collectively regulate the male meiotic spindle, cytokinesis and midbody abscission, in addition to diverse spermatid remodelling events, including Golgi organisation, and acrosome and manchette formation. We also define KATNAL1-specific roles in sperm flagellum development, manchette regulation and sperm-epithelial disengagement. Finally, using proteomic approaches, we define the KATNA1, KATNAL1 and KATNB1 mammalian testis interactome, which includes a network of cytoskeletal and vesicle trafficking proteins. Collectively, we reveal that the presence of multiple katanin A-subunit paralogs in mammalian spermatogenesis allows for 'customised cutting' via neofunctionalisation and protective buffering via gene redundancy.