Spindle assembly factor. Required for normal assembly of mitotic spindles. Mediates the binding kif15 and aurka to spindle microtubules. Required for targeting kif15 to microtubule minus ends. Activates aurka by promoting its autophosphorylation and protects the phosphorylated residue against dephosphorylation (By similarity). Associates with microtubules. Interacts with aurka and plk1. Interacts with kif15 (By similarity). Localizes during metaphase on spindle microtubules, with a strong enrichment at spindle poles. Localizes to the minus ends of spindle pole and aster microtubules in a dynein- and dynactin-dependent manner (By similarity). Phosphorylated during mitosis. Hyperphosphorylated upon assembly of microtubules (By similarity). Belongs to the TPX2 family.