Sabrina Valle & Ben Klayman

HOUSTON (Reuters). Exxon Mobil Corp (NYSE:). On Tuesday, Dan Ammann (ex-president of General Motors) was appointed to head the company’s Energy Transition Business. He will be effective May 1.

This appointment is a sign that Exxon will hire more outsiders to fill senior positions after activist investors took three of the U.S. producer board’s seats last year. Kathryn Mikells was a former drinks producer employee that Exxon hired last year. Diageo (LON. ) Plc is its chief finance officer.

Exxon announced Tuesday that Joe Blommaert has been replaced by Ammann as the chief of Low Carbon Solutions. Blommaert, who has been with the oil producer for 35 years, will be retiring.

Ammann, who was the chief executive officer of Cruise’s self-driving vehicle subsidiary, abruptly resigned from Detroit Automaker in December. Prior to running Cruise, Ammann served as the Detroit company’s president and chief financial officer, and was a Wall Street banker with Morgan Stanley (NYSE:) Before that.

Sources close to the story claim that he was fired for disagreeing with GM about Cruise’s public offering. Mary Barra, GM Chief Executive, has minimized the need to make Cruise public quickly. The automaker has also stressed how important that business is for its operations.

Ammann couldn’t be reached immediately to comment.

In 2010, he joined GM and was appointed treasurer. He also helped to lead the initial public offering of the company following its restructuring in 2009.

Exxon will have him as its chief executive officer for the energy transition division. He is responsible for generating new revenue through carbon sequestration and hydrogen production. Exxon, as part its restructuring announced in January, put the unit on equal footing with other operations.

“We welcome Dan to ExxonMobil and will use his knowledge and experience to continue to build our Low Carbon Solutions business,” said Exxon Chief Executive Darren Woods.