Component of the kinetochore, a multiprotein complex that assembles on centromeric DNA and attaches chromosomes to spindle microtubules, mediating chromosome segregation and sister chromatid segregation during meiosis and mitosis (By similarity). Component of the inner kinetochore COMA subcomplex, which connects centromere-associated proteins and the outer kinetochore (PubMed:29046335). COMA interacts with other inner kinetochore proteins to form the inner kinetochore constitutive centromere-associated network (CCAN), which serves as a structural platform for outer kinetochore assembly (By similarity). Component of the heterotetrameric kinetochore subcomplex COMA, which consists of AME1, CTF19, MCM21 and OKP1 (PubMed:22322944, PubMed:29046335). The COMA subcomplex is part of a larger constitutive centromere-associated network (CCAN) (also known as central kinetochore CTF19 complex in yeast), which is composed of at least AME1, CHL4, CTF3, CTF19, IML3, MCM16, MCM21, MCM22, NKP1, NKP2 and OKP1 (By similarity). COMA binds the centromeric nucleosome-binding protein MIF2, and to the outer kinetochore MIND subcomplex (PubMed:29046335). The D-RWD region contains a double-RWD domain which is involved in the interaction with other kinetochore proteins. Belongs to the CENP-O/MCM21 family.