Component of the central kinetochore; which mediates the attachment of the centromere to the mitotic spindle by forming essential interactions between the microtubule-associated outer kinetochore proteins and the centromere-associated inner kinetochore proteins. Required for establishing bipolar spindle- microtubule attachments and proper chromosome segregation. {ECO:0000269|PubMed:11361335; ECO:0000269|PubMed:15241623}.