General Motors CEO Ed Whitacre will step down Sept. 1, with GM board member Dan Akerson becoming the fourth GM chief in two years. Whitacre, 68, will stay on as chairman until the end of the year. UPDATE.

Akerson, a top executive at buyout firm The Carlyle Group, built a career in telecommunications, as CEO of Nextel and XO Communications. He was one of the GM board members appointed by the U.S. Treasury to watch over its 61% stake in GM, a stake its expected to begin selling off when GM begins selling its shares publicly later this year.

Whitacre said he had always considered his tenure at GM as limited to getting the company launched out of bankruptcy.

"The board has been aware of this since the day I became CEO," he said. "It was my public duty to help return this company to greatness, and that I didn't want to stay a day beyond that, really."

Akerson will also take the title of chairman from Whitacre on Dec. 31.