The 26S proteasome is composed of two large subcomplexes. The 20S core is barrel shaped, and proteolysis takes place inside its chamber. The 19S cap are regulatory subcomplexes which can be further divided into the base and the lid. The 19S cap is essential for proper functioning of the proteasome; however, its precise function is not clear. One of the subunits in the 19S cap, Rpn10 (n10), can bind polyubiquitin, suggesting that one of the functions of the 19S cap is to bind polyubiquitinated substrates. Several proteins in the base are ATPases (those labeled with a t), which are thought to play a role in denaturing protein substrate such that they can enter the narrow opening of the catalytic core. Securin and mitotic cyclin are two of the best documented substrates whose levels are tightly regulated by the proteasome. Improper degradation of securin can block chromosome segregation because securing regulates the removal of the "glue" protein that binds the two sister chromatids together. Inefficient proteolysis of cyclin blocks cell division in anaphase.