of the ATPases is a dynein-like Rea1 protein made up of 6 different ATPase domains that form a ring structure. The ring structure is attached to a flexible tail that happens to have a MIDAS (Metal ion-dependent adhesion site) tip. The Rea1 interacts with the 60S pre-ribosome via its ring while two substrates, Ytm1 and Rsa1, interact with Rea1 through its MIDAS tip. The role of these substrates has not yet been defined. Both though, along with their interactions, are removed in the maturation process of the 60S pre-ribosome. The other two ATPases, Rix7 and Drg1 also function to remove assembly factors from the maturing 60S subunit. Helicases and GTPases are also involved in the removal of assembly factors and the rearrangement of RNA to form the completed 60S subunit. Once in the cytoplasm (see nuclear export), the 60S subunit further undergoes processing in order to be functional. The rest of the large subunit ribosomal particles associate with the 60S unit and the remaining non-ribosomal assembly factors disassociate. The release of the biogenesis factors is mediated mostly by GTPases such as Lsg1 and ATPases such as Drg1. The precise sequence of these events remains unclear. The pathway of 60S cytoplasmic maturation remains incomplete as far as current knowledge is concerned. Nuclear export In order for the pre-ribosomal units to fully mature, they must be exported to the cytoplasm. To effectively move from the nucleolus to the cytoplasm, the pre-ribosomes interact with export receptors to move through the hydrophobic central channel of the nuclear pore complex. The karyopherin Crm1 is the receptor for both ribosomal subunits and mediates export in a Ran-GTP dependent fashion. It recognizes molecules that have leucine-rich nuclear export signals. The Crm1 is pulled to the large 60S subunit by the help of an adapter protein called Nmd3. The adapter protein for the 40S unit is unknown. In addition to Crm1, other factors play a role in nuclear export of pre-ribosomes. A general mRNA export receptor, called Mex67, as well as a HEAT-repeating-containing protein, Rrp12, facilitate the export of both subunits. These factors are non-essential proteins and help to optimize the export of the pre-ribosomes since they are large molecules. Quality control Because ribosomes are so complex, a certain number of ribosomes are assembled incorrectly and could potentially waste cellular energy and resources when synthesizing non-functional proteins. To prevent this, cells have an active surveillance system to recognize