The Cortical Protein Num1p Is Essential for Dynein-Dependent Interactions of Microtubules with the Cortex

Abstract

In budding yeast, the mitotic spindle moves into the neck between the mother and bud via dynein-dependent sliding of cytoplasmic microtubules along the cortex of the bud. How dynein and microtubules interact with the cortex is unknown. We found that cells lacking Num1p failed to exhibit dynein-dependent microtubule sliding in the bud, resulting in defective mitotic spindle movement and nuclear segregation. Num1p localized to the bud cortex, and that localization was independent of microtubules, dynein, or dynactin. These data are consistent with Num1p being an essential element of the cortical attachment mechanism for dynein-dependent sliding of microtubules in the bud.

The role of mitochondria in anchoring dynein to the cell cortex extends beyond clustering the anchor protein

“…In addition, the synthetic system creates stable clusters that decrease the dynamics of Num1 at the plasma membrane, which may also favor mitochondrial tethering. A similar scenario is observed for wildtype Num1 clusters, which once assembled are stationary, display limited exchange with the non-assembled pool, and persistently tether mitochondria for extended periods of time [5,17]. Interestingly, only a subset of PAN clusters tether mitochondria.…”

“…We next examined if PAN clusters were able to anchor dynein to the cell cortex, which is required for dynein-mediated nuclear inheritance [5]. Yeast have two partially redundant nuclear positioning pathways: the dynein- and Kar9-mediated pathways [2,25,26].…”

“…The CC domain interacts directly with the mitochondrial membrane and is also required for the Num1-dynein interaction [10–12]. The PH domain, which binds with high specificity to PI 4,5 P 2 , targets Num1 to the plasma membrane [13,14], where the protein is found in clusters as well as in a pool that is diffusely localized along the plasma membrane and with cortical ER [3,5,10,15,16]. Cluster formation, which is dependent on the CC domain and an interaction with mitochondria (Figure 1(a)), is critical for the function of Num1 in both mitochondria and dynein anchoring [10,11,17].…”

The Centrosome and Its Duplication Cycle

“…In one, Kar9 associates with both the microtubule plus end-binding protein EB1 and the myosin motor protein Myo2 to guide astral microtubules along actin filaments that emanate from the incipient bud site and growing bud tips (Beach et al 2000;Lee et al 2000;Yin et al 2000). In the second pathway, dynein both guides the astral microtubules along the cell cortex and promotes instability of the plus ends to ensure end-on association of microtubules with the cortex (McMillan and Tatchell 1994;Carminati and Stearns 1997;Adames and Cooper 2000;Heil-Chapdelaine et al 2000;Yeh et al 2000). When astral guidance fails, the ensuing nuclear division within the mother cell blocks cytokinesis and mitotic exit through activation of the spindle orientation checkpoint (SPOC) (Yeh et al 1995;Bardin et al 2000;Pereira et al 2000;Caydasi and Pereira 2012).…”

A nuclear contortionist: the mitotic migration ofMagnaporthe oryzaenuclei during plant infection

“…For example, in S. cerevisiae , astral microtubules are nucleated at the spindle pole body and rely on dynamic instability to search the cell cortex of the bud to locate Num1, a cortical dynein-interacting protein (Carminati and Stearns 1997; Heil-Chapdelaine et al 2000; Farkasovsky and Küntzel 2001). Once the plus-ends of astral microtubules bind to Num1, they slide against the cell cortex in a dynein-mediated manner which moves the spindle and eventually the daughter nucleus into the newly formed bud (Heil-Chapdelaine et al 2000). Functional homologs of Num1 are also required for proper nuclear migration in multinucleate fungi, including A. nidulans (Veith et al 2005) and Ashbya gossypii (Grava et al 2011).…”