Ndc80 Loop as a protein-protein interaction motif

Abstract

Our understanding of the structure and function of kinetochores has advanced dramatically over the past 10 years, yet how the plus end of spindle microtubules interacts with the kinetochore and establishes amphitelic attachment for proper sister chromatid segregation remains unresolved. However, several recent reports from different organisms have shed new light on this issue. A key player in microtubule-kinetochore interaction is the conserved Ndc80 outer kinetochore complex. In both yeast and human cells in particular, a ubiquitous internal ‘loop’ found in the Ndc80 molecule interrupting its C-terminal coiled-coil domain plays critical roles in protein-protein interaction, by recruiting microtubule-binding proteins to ensure proper kinetochore-microtubule attachment. In this commentary, we summarise the recent progress made and discuss the evolutionary significance of this loop’s role in microtubule dynamics at the kinetochore for accurate chromosome segregation.

The internal loop of fission yeast Ndc80 binds Alp7/TACC-Alp14/TOG and ensures proper chromosome attachment

“…It has become clear that the Ndc80 loop plays a common role among eukaryotes as a protein–protein interaction site in recruiting other proteins to the mitotic outer kinetochore (Nilsson, 2012; Varma and Salmon, 2012; Tang and Toda, 2013). In human cells, a DNA replication licensing factor Cdt1 (Machida et al ., 2005; Sclafani and Holzen, 2007) and the Ska1 complex (also called the Ska complex; Guimaraes and Deluca, 2009) are recruited to the kinetochore directly or indirectly via the Ndc80 loop, and this step is critical for establishing proper spindle–kinetochore attachment (Chan et al ., 2012; Jeyaprakash et al ., 2012; Schmidt et al ., 2012; Varma et al ., 2012; Zhang et al ., 2012).…”

“…Although this loop may play a structural role, such as in the introduction of the coiled-coil kink to the rod region of the Ndc80 complex (Wang et al ., 2008; Wan et al ., 2009; Varma et al ., 2012; Zhang et al ., 2012), most recent results from yeasts and humans have shown that this domain also acts as a protein–protein interaction motif (Hsu and Toda, 2011; Tang and Toda, 2013; Maure et al ., 2011; Nilsson, 2012; Varma et al ., 2012; Varma and Salmon, 2012; Zhang et al ., 2012). We previously showed that the Ndc80 loop in fission yeast recruits the Dis1 microtubule-associated protein (Ohkura et al ., 1988; Nabeshima et al ., 1995) to the mitotic outer kinetochore via direct interaction (Hsu and Toda, 2011).…”

Introduction to Fission Yeast as a Model System

“…Partially condensed chromosomes become aligned on the spindle through attachment at the kinetochore (Hiraoka et al 1984;SanchezPerez et al 2005;Tang and Toda 2013), and an actinomyosin band forms over the nucleus defining the future site of cytokinesis (Lee et al 2012). Once correctly aligned, sister chromatids segregate to opposite poles of the elongating spindle, followed by spindle breakdown (Watanabe 2010;Hsu and Toda 2011).…”

Mapping the kinetochore MAP functions required for stabilizing microtubule attachments to chromosomes during metaphase

“…The loop primarily imparts some degree of flexibility by introducing a break in the otherwise long and rigid coiled‐coil structure of Ndc80 (Nilsson, ). Although the loop does not contribute to MT‐binding directly and is separated by distance from the globular Hec1/Nuf2 heads that partake in MT‐binding activity, it is interesting that there has been a recent upsurge in the study of this internal loop region, particularly its role in establishing end‐on attachments with MTs for accurate chromosomal segregation (Nilsson, ; Tang & Toda, ; Zhang et al, ). Thus, it is imperative to understand the intricacies of how the loop domain of Ndc80 participates in the modulation of spindle MTs during cell division.…”

“…There is a plethora of evidence available from studies on organisms from yeast to humans suggesting that the loop domain of Ndc80 functions as a docking hub for several proteins that can bind to MTs at kinetochores (Hsu & Toda, ; Maure et al, ; Tang & Toda, ; Varma et al, ; Zhang et al, ): for example, the Dam1/DASH complex in budding yeast, Dis1 in fission yeast, and the Ska complex and Cdt1 in humans are all important components recruited to kinetochores with the assistance of the loop domain. A common theme that emerges from these findings is that the loop domain may function to recruit several MT‐binding proteins to kinetochores where they can optimally augment the MT‐binding ability of the Ndc80 complex.…”