"The space shuttle Discovery was transported to the Vehicle Assembly Building at NASA's Kennedy Space Center in Florida on Saturday, where it will be fitted with an external fuel tank and two solid rocket boosters," Roscosmos said on its website.
The 13-day mission is the second of three flights to deliver parts from the Japanese Kibo laboratory to the ISS. Primary payloads are the second pressurized module and the lab's robotic arm system.
Space shuttle Endeavour delivered the first pressurized component of the lab to the orbital station in March.
U.S. astronaut Mark Kelly will command the seven-member crew, which includes Japan Aerospace Exploration Agency (JAXA) astronaut Akihiko Hoshide.
The crew is expected to conduct at least two spacewalks during the mission.