Hmg1-mCherry was used to visualize the NE (Fig. Nsp1 releases a barrier that otherwise prevents NPC passage through the bud neck. It further supports the finding that NPC inheritance, not de novo NPC assembly, is primarily responsible for controlling NPC number in daughter cells. Introduction Asymmetric cell divisions are critical for cell fate determination during embryogenesis, organogenesis, and differentiation (Neumller and Knoblich, 2009). Since the budding yeast undergoes an asymmetric division, it is an effective model for identifying factors that are actively segregated along the polarity axis and the underlying molecular mechanisms responsible for their segregation (Pruyne et al., 2004). In yeast, the two type V myosin motors Myo2 and HDAC6 Myo4 deliver organelles, secretory vesicles, and mRNAs to the daughter cell (Pruyne et al., 2004; Chung and Takizawa, 2010; Eves et al., 2012). Myo2 also plays a role in nuclear migration by guiding spindle microtubules along actin cables in concert PF-04979064 with a complex of proteins at the plus ends of microtubules, including Kar9 and Bim1 (Korinek et al., 2000; Miller et al., 2000; Yin et al., 2000). A redundant nuclear positioning pathway requires the dyneinCdynactin complex (Eshel et al., 1993; Li et al., 1993; Grava et al., 2006). Recent data also implicates the exocyst complex in anchoring ER tubules that extend from the mother nuclear envelope (NE) to the bud cortex in maintaining nuclear position at the bud neck (Kirchenbauer and Liakopoulos, 2013). Further, the ubiquitylation of a component of the nuclear pore complex (NPC) was shown to function in nuclear migration through the recruitment of dynein light chain to the NE (Hayakawa et al., 2012). The latter process reflects several connections uncovered between NPCs and the cytoskeleton (Stelter et al., 2007; Splinter et al., 2010; Bolhy et al., 2011; Steinberg et al., 2012). NPCs are massive protein assemblies embedded in the NE that control the flux of molecules between the nucleus and cytoplasm. Each NPC PF-04979064 is composed of 30 individual protomers termed nucleoporins (nups; Rout et al., 2000; Cronshaw et al., 2002) found in distinct subcomplexes (Siniossoglou et al., 1996, 2000; Grandi et al., 1997; Marelli et al., 1998; Belgareh et al., 2001; Vasu et al., 2001; Alber et al., 2007a,b; Onischenko et al., 2009). These subcomplexes are thought to form modular building blocks that contribute to the formation of the concentric membrane, inner and outer ring complexes that surround a central transport channel (Alber et al., 2007a,b). The channel itself is rich in unstructured nups like Nsp1/Nup62 that contain repetitive peptide motifs of Phe-Gly (FG-nups; Alber et al., 2007a,b). Nsp1 helps form two subcomplexes at the NPC composed of Nup49, Nup57, and Nic96, or Nup82 and Nup159 (Nehrbass et al., 1990; Mutvei et al., 1992; Grandi et al., 1995; Schlaich et al., 1997; Bailer et al., 2000, 2001). Because transport through NPCs is essential for cell life, there are likely mechanisms to ensure that NPC numbers can accommodate cell typeCspecific nuclear transport loads. We understand little about mechanisms that control NPC number. Lymphocyte stimulation results in a doubling of NPC number, which suggests that external inputs can up-regulate the NPC assembly pathway (Maul et al., 1972). Further, the S-phase doubling of NPCs observed in cell culture suggests that NPC assembly is linked to the cell cycle (Maul et al., 1972), perhaps through cyclin-dependent kinases (Maeshima et al., 2010). Mutations in nups important for NPC assembly PF-04979064 can also impact differentiation programs (Lupu et al., 2008; de Jong-Curtain et al., 2009; DAngelo et al., 2012). These studies cumulatively suggest that NPCs themselves might be important for cell fate determination and underscore the importance of identifying mechanisms that control NPC number. One way to modulate NPC number is to regulate the de novo assembly of NPCs, which occurs by postmitotic and interphase mechanisms (Doucet et al., 2010). During de novo NPC assembly in interphase, the membrane and inner ring complexes assemble at the NE first and might directly contribute to fusion of the inner and outer nuclear membranes (Makio et al., 2009; Onischenko et al., 2009; Doucet et al., 2010; Dultz and Ellenberg, 2010; Fichtman et al., 2010; Vollmer et al., 2012). Other conserved.