The gamma-Tubulin complex is composed of gamma Tubulin and the gamma-Tubulin complex-associated proteins GCP2, GCP3, GCP4, GCP5 and GCP6, all of which are essential components of microtubule organizing centers. gamma-Tubulin complex components are localized to both the centrosome, where they are involved in microtubule nucleation, and to the cytoplasm, where they exist as soluble complexes that can be recruited to the centrosome as needed. Although the GCP proteins are related, they have distinct roles which contribute to the proper function of the gamma-Tubulin complex. GCP4 (gamma-tubulin complex component 4), also known as TUBGCP4, is a ubiquitously expressed 667 amino acid member of the gamma-Tubulin complex that localizes to the metaphase spindle during mitosis. In response to proteosome inhibition, GCP4 exhibits increased accumulation at the pericentiolar material where it participates in microtubule organization and nucleation.