Quote from the article, link below.
Taking the heft out:
•Lighter materials. The Infiniti EX35, which goes on sale in December, has a plastic rear hatch and aluminum bumper supports, suspension components, hood and wheels. Infiniti parent Nissan says the goal was to make the luxury sedan weigh about 60 pounds less than comparable BMW and Lexus models.
Some high-temperature plastics can be used for engine parts such as valve covers.