FAR 121.437
No pilot may act as pilot in command of an aircraft (OR A SECOND IN COMMAND OF AN AIRCRAFT IN A FLAG OR SUPPLEMENTAL OPERATION THAT REQUIRES 3 OR MORE PILOTS) unless he holds an airline transport pilot certificate and AN APPROPRIATE TYPE RATING for that aircraft.
Also, in order to certify a carrier to fly certain routes, the FAA requires crewmember to undergo certain training, and in some routes the SIC must be typed.
I've jumped to Europe a couple of times on US airlines, and in both of them the FOs were typed. Most carriers have 2 FOs and 1 CA, so how can the CA take a brake if the FO is not rated (I don't know if that can be done)?
PEACE