Binds to the plus end of microtubules and regulates microtubule dynamics and microtubule organization. May play a role in the attachment, organization, and/or dynamics of microtubule ends at the spindle pole body. Stabilizes both cytoplasmic and nuclear microtubules. Promotes mitotic spindle elongation in anaphase. Has microtubule polymerase activity by accelerating microtubule growth and inhibiting catastrophe. The polymerase activity is proposed to involve a tethering mechanism at the microtubule plus end: a curved, longitudinally microtubule lattice-associated tubulin heterodimer (that cannot be incorporated into the microtubule lattice) is bound via one TOG domain while the other TOG domain binds to an unpolymerized tubulin heterodimer leading to lateral association of these tubulin heterodimers; polymerization-induced straightening of the tubulin heterodimer releases the polymerase (PubMed:11309422, PubMed:25172511, PubMed:16567500, PubMed:25097237). Is involved in regulation of kinetochore-microtubule attachments in a tension-dependent manner involving its association with the NDC80 complex; the function may be independent of its microtubule polymerase activity. Can either stabilize or destabilize kinetochore attachments, depending on the level of kinetochore tension and whether the microtubule tip is assembling or disassembling (PubMed:27156448). Binds to microtubules. Homodimer; each monomer can bind via its TOG domains to two alpha/beta-tubulin heterodimers. Interacts with SPC72. Associates with the NDC80 complex. Localizes primarily to the spindle pole body (SPB) and to a lesser extent along spindle microtubules. The coiled coil domain mediates homodimerization. The TOG (tumor overexpressed gene) domains are composed of six (for the most part non-canonical) HEAT repeats each. Intra-HEAT loops are positioned along a face of the TOG domain and bind to a single alpha/beta-tubulin heterodimer. Two sets of TOG domains may wrap around a single free tubulin heterodimer accompanied by a open-to-closed conformational change of the STU2 homodimer. Both, TOG 1 and TOG 2 bind curved alpha/beta-tubulin with comparable affinity; the two TOG domains bind noncooperatively to two tubulin heterodimers. Free unpolymerized tubulin-binding is predominantly mediated by TOG 1, association with the microtubules plus ends is mediated by TOG 2 in cooperation with a C-terminal basic domain. Mutations in STU2 suppress a cold-sensitive mutation in TUB2. Present with 1660 molecules/cell in log phase SD medium. Belongs to the TOG/XMAP215 family.