 The entry and exit of large molecules from the nucleus is tightly controlled by the nuclear pore complexes . Although small molecules can enter the nucleus without regulation , macromolecules such as RNA and proteins require association karyopherins called importins to enter the nucleus and exportins to exit . " Cargo " proteins that must be translocated from the cytoplasm to the nucleus contain short amino acid sequences known as nuclear localization signals , which are bound by importins , while those transported from the nucleus to the cytoplasm carry nuclear export signals bound by exportins . The ability of importins and exportins to transport their cargo is regulated by GTPases , enzymes that hydrolyze the molecule guanosine triphosphate to release energy . The key GTPase in nuclear transport is Ran , which can bind either GTP or GDP ( guanosine diphosphate ) , depending on whether it is located in the nucleus or the cytoplasm . Whereas importins depend on RanGTP to dissociate from their cargo , exportins require RanGTP in order to bind to their cargo . 
