Essential component of the mitotic checkpoint; which prevents cells from prematurely exiting mitosis. Required for the assembly of the dynein-dynactin and MAD1-MAD2 complexes onto kinetochores. Involved in regulation of membrane traffic between the Golgi and the endoplasmic reticulum. {ECO:0000269|PubMed:11590237; ECO:0000269|PubMed:15029241; ECO:0000269|PubMed:15094189; ECO:0000269|PubMed:15485811; ECO:0000269|PubMed:15824131}.