Proteins perform a variety of mechanical functions including those that produce cell movement and muscle contraction, as well as the transport of molecules between different locations inside and outside of a cell. Mechanical protein functions are powered by the conversion of chemical energy to mechanical work through conformational changes in protein structure. Within a protein domain, simple chemical changes, like the hydrolysis of bound molecules such as ATP or GTP, can result in conformational changes that create much larger movements in the protein. ATP hydrolysis powers the motor protein myosin, which acts as a lever, pulling actin filaments and causing muscles to contract. Helicases that unwind DNA during replication and transcription also use ATP to break hydrogen bonds and move along the DNA strand. In the ribosome, many proteins work together as a machine to synthesize new proteins. GTP hydrolysis allows the elongation factor, EF-Tu to transfer a tRNA molecule to the ribosome so it can add an amino acid to the growing protein strand. tRNA is associated with EF-Tu when GTP is bound. When GTP is hydrolyzed to GDP, the release of the phosphate group results in a small conformation change in and near the nucleotide-binding site. This small shift causes an α helix located at the interface of the GTPase domain and the other two domains to change positions. The movement of this helix causes two domains to swing open to release the bound tRNA.