Structural and Functional Organization of the Ska Complex, a Key Component of the Kinetochore-Microtubule Interface

Abstract

The Ska complex is an essential mitotic component required for accurate cell division in human cells. It is composed of three subunits that function together to establish stable kinetochore-microtubule interactions in concert with the Ndc80 network. We show that the structure of the Ska core complex is a W-shaped dimer of coiled coils, formed by intertwined interactions between Ska1, Ska2, and Ska3. The C-terminal domains of Ska1 and Ska3 protrude at each end of the homodimer, bind microtubules in vitro when connected to the central core, and are essential in vivo. Mutations disrupting the central coiled coil or the dimerization interface result in chromosome congression failure followed by cell death. The Ska complex is thus endowed with bipartite and cooperative tubulin-binding properties at the ends of a 350 Å-long molecule. We discuss how this symmetric architecture might complement and stabilize the Ndc80-microtubule attachments with analogies to the yeast Dam1/DASH complex.

Anaphase A: Melting Microtubules Move Chromosomes toward Spindle Poles

“…Other molecules and structures could harness curling protofilaments. In humans and other eukaryotes, for example, the Ska complex might act as an attachment-stabilizer in a manner similar to Dam1c [121,122,165]. Ska complex does not appear to form oligomeric microtubule-encircling rings, but like Dam1c it can track with disassembling tips [166].…”

“…Ska complex does not appear to form oligomeric microtubule-encircling rings, but like Dam1c it can track with disassembling tips [166]. The Ska complex also dimerizes and might form lateral bridges between neighboring Ndc80 complexes [122]. Curling protofilaments might hook these lateral bridges.…”

“…Certain structural features of kinetochore subcomplexes also seem ideal for biased diffusion. Ndc80c [104], Dam1c [164], Ska complex [122], and Cenp-F [170] all appear to bind microtubules through flexible domains, which could allow some to bear load while others unbind and rebind in new locations, enabling a kinetochore to move or reorient on the microtubule without detaching. Diffusion along the microtubule lattice is negligibly slow for large assemblies of Dam1c [152] and for whole native kinetochore particles [43], but these observations do not rule out biased diffusion as a mechanism for tip-coupling by these assemblies.…”

Building an integrated model of chromosome congression

“…Thus, the Ska complex represents the current best in vitro candidate for a molecule that directly binds protofilaments as they dissemble. Nevertheless, the in vivo functions of the Ska complex are somewhat unclear: initial studies reported either a prolonged mitotic delay with a few misaligned kinetochores (Abad et al, 2014;Hanisch et al, 2006;Jeyaprakash et al, 2012;Theis et al, 2009) or a more severe congression defect and associated changes in the stability of microtubule-kinetochore attachment Gaitanos et al, 2009;Schmidt et al, 2012;Welburn et al, 2009). The latter phenotype is most likely due to cohesion fatigue (the asynchronous separation of sister chromatids), which occurs downstream of a prolonged metaphase delay (Daum et al, 2009;Sivakumar et al, 2014).…”

Divalent Cations Affect the Stability and Structure of Dad2p, a Subunit of the Candida Albicans Kinetochore Dam1 Complex

“…This difference has been postulated to reflect the more stringent requirement for maintaining connection with the depolymerizing microtubule when a single point of attachment is present; thus, under these circumstances, the ring may provide necessary stability (Burrack et al, 2011;Thakur & Sanyal, 2011). Outside of the yeasts, sequence-based homologues of the Dam1 complex proteins are not identifiable, though a variety of evidence suggests that the functional homologue in metazoans may be the Ska complex (Gaitanos et al, 2009;Guimaraes & Deluca, 2009;Hanisch, Sillje, & Nigg, 2006;Jeyaprakash et al, 2012;Welburn et al, 2009). …”