The Astrin/SKAP complex plays important roles in mitotic chromosome alignment and centrosome integrity, but previous work found conflicting results for SKAP function. molecular players associate with the cell division apparatus to facilitate spindle assembly and chromosome segregation. Previous work from our laboratory and others identified the Astrin/SKAP complex (Schmidt et al., 2010; Dunsch et al., 2011), which comprises Astrin (also referred to as Spag5), the dynein light chain LC8, and the small kinetochore-associated protein SKAP/KNSTRN (Fang et al., 2009; also referred to as C15orf23, Traf4af1, or Kinastrin). The Astrin/SKAP complex is usually highly expressed in mitosis (Whitfield et al., 2002; Fang et al., 2009; Thiru et al., 2014), where it localizes to aligned kinetochores and the mitotic spindle and plays multiple important functions, including in chromosome alignment and the maintenance of spindle bipolarity (Mack and Compton, 2001; Gruber et al., 2002; Thein et al., 2007; Manning et al., 2010; Schmidt et al., 2010; Dunsch et al., 2011). Although SKAP plays a central role within this complex, previous work found conflicting results for its functions and behavior. Here, we find that the SKAP isoform used in all previous studies of the human protein is usually exclusively expressed in mammalian testes, whereas HKI-272 mitotic cells instead express a shorter SKAP isoform. Our analysis of the mitotic SKAP isoform discloses a striking localization of this protein along the length of spindle microtubules and to microtubule plus ends, including to astral microtubules, suggesting potential functions for this complex beyond its previously defined functions in chromosome segregation. Microtubules emanating from the spindle poles interact with two major subcellular sites: kinetochores and the cell cortex. Whereas kinetochores link microtubules to chromosomal DNA to direct chromosome segregation, the cell cortex anchors astral microtubules to the plasma membrane to generate cortical pulling causes that direct spindle positioning and orientation. Spindle positioning is usually crucial for organismal development and cellular viability (G?nczy, 2008; Siller and Doe, 2009; Knoblich, 2010). The position of the mitotic HKI-272 spindle within a dividing cell establishes the cell division plane and the site of the cytokinetic furrow, thereby determining the comparative sizes of the two daughter cells. The pressure to move the spindle within a cell is usually generated by the conversation of astral microtubule plus ends with CDF the microtubule-based motor cytoplasmic dynein, which is usually localized to the cell cortex (Kiyomitsu and Cheeseman, 2012; Kotak et al., 2012; McNally, 2013; Kiyomitsu, 2015). Astral microtubules are a unique mitotic populace of highly dynamic microtubules that originate from the centrosome and grow toward the cell cortex. When astral HKI-272 microtubules contact the cortex, dynein is usually thought to establish an end-on attachment and generate pulling pressure to move the spindle toward the cell cortex (Hendricks et al., 2012; Laan et al., 2012). The amount of pulling pressure on each side of the spindle is usually regulated through dynamic changes in the comparative levels of cortical dynein (Collins et al., 2012; Kiyomitsu and Cheeseman, 2012). As a cell progresses from prometaphase into metaphase, the dynein motors on each side of the cell engage in a brief tug-of-war until the spindle is usually positioned at the cell center. In human cells, mitotic spindle position is usually controlled by both extrinsic and intrinsic cues (Fink et al., 2011; Kiyomitsu and Cheeseman, 2012). Much of the work on spindle positioning has focused on external or cortical factors, leaving open important questions regarding the function of astral microtubules. Although several.