During mitosis, the segregation of chromosomes into newly forming daughter cells is orchestrated by the microtubule spindle. γ-tubulin is the key protein for nucleation of microtubules that emanate from centrosomes, chromosomes and from pre-existing microtubules. The latter process is also controlled through the Augmin complex. However, how both γ-tubulin and Augmin are recruited to the mitotic spindle are not completely understood. In this issue (Larsson et al., 2018), Einar Hallberg and co-workers investigate the involvement of spindle-associated membrane protein 1 (Samp1), an inner nuclear transmembrane protein, in spindle assembly. The authors show that depletion of Samp1 leads to cells having a prolonged metaphase and an increase in chromosome mis-segregation. This loss of Samp1 is accompanied by a decrease in β-tubulin and γ-tubulin levels in spindles. A Samp1 fusion protein localises to the spindle, confirming earlier results, and Samp1 partially colocalises with γ-tubulin. Importantly, Samp1 binds γ-tubulin directly and is found in a complex with the HAUS6 subunit of the Augmin complex, which is also partially lost from the spindle after Samp1 depletion. This study sheds light on the mechanisms of γ-tubulin recruitment and spindle assembly for correct chromosome segregation in dividing cells to avoid aneuploidy, a hallmark in cancer.
