KNL1: bringing order to the kinetochore

Abstract

KNL1 is an evolutionarily conserved kinetochore-associated protein essential for accurate chromosome segregation in eukaryotic cells. This large scaffold protein, predicted to be almost entirely unstructured, is involved in diverse mitotic processes including kinetochore assembly, chromosome congression, and mitotic checkpoint signaling. How this kinetochore "hub" coordinates protein-protein interactions spatially and temporally during mitosis to orchestrate these processes is an area of active investigation. Here we summarize the current understanding of KNL1 and discuss possible mechanisms by which this protein actively contributes to multiple aspects of mitotic progression.

Figure 1. Domain architecture of human KNL1

Key regions involved in KNL1 function are indicated schematically (top), and the corresponding amino acid sequences for each region in human KNL1 are listed (bottom). Aurora B kinase phosphorylates the N-terminus of KNL1 to inhibit association with PP1 (Liu et al. 2010). Mps1 phosphorylates the KNL1 MELT repeats to promote association with Bub1 and Bub3 (London et al. 2012; Primorac et al. 2013; Shepperd et al. 2012; Yamagishi et al. 2012). Amino acid residues 1-68 indicate the MT binding domain (Welburn et al. 2010), and underlined is a basic patch of residues (RRRH), whose analogous residues in C. elegans have been implicated in direct MT interaction (Espeut et al. 2012).

Figure 2. Evolutionary conservation of KNL1 domain architecture

Key KNL1 functional regions are shown for several organisms. The number of amino acids in KNL1 for the various organisms are shown (right). Two values in this column indicate multiple isoforms (H. sapiens). The green region in D. melanogaster indicates the presence of repetitive motifs that differ from the MELT consensus motif observed in other organisms (Schittenhelm et al. 2009).

Figure 3. KNL1-dependency of kinetochore protein localization

Solid arrows indicate a high level of dependence for localization and dashed arrows indicate a partial dependence. Arrows do not indicate direct interaction.

Figure 4. Proposed model for the function of KNL1 in kinetochore-MT attachment regulation and the SAC

In early mitosis, KNL1 promotes the kinetochore recruitment of SAC proteins Bub1, BubR1, Bub3, Mad1, and Mad2, which leads to generation and amplification of the “wait anaphase” signal that inhibits APC/C^Cdc20 activation. Association of Bub1 with KNL1 promotes Bub1 activity, leading to centromere and kinetochore Aurora B kinase recruitment and activation. Aurora B phosphorylates outer kinetochore substrates, which prevents premature kinetochore-MT stabilization. In late mitosis, KNL1-mediated recruitment of phosphatases antagonizing Aurora B promotes kinetochore-MT attachment stability. PP1 dephosphorylates KNL1, resulting in Bub1 dissociation and subsequent loss of SAC proteins. PP1 binding to KNL1 and KNL1 MT binding additionally promotes SAC silencing. With attached kinetochores no longer amplifying the “wait anaphase” signal, dissembled SAC inhibitory complexes lead to APC/C^Cdc20 activation, Cyclin B and securin degradation, and mitotic exit. *The cartoon shows the Mis12 complex and CENP-T simultaneously binding to the Ndc80 complex for simplicity. However, it has been demonstrated that these two kinetochore components compete for binding to the Ndc80 complex (Bock et al. 2012; Nishino et al. 2013; Schleiffer et al. 2012).