Detroit - General Motors chief executive Fritz Henderson abruptly resigned on Tuesday, after the company's board decided it wanted to chart a new course for the restructuring automaker.Henderson was asked by the board to step down at a meeting in Detroit after being on the job for just eight months, according to a person with direct knowledge of the matter.GM chairman Ed Whitacre, 68, will become interim chief executive as the automaker begins an immediate search for a replacement, the company said.The announcement of Henderson's sudden departure underscored the tough oversight being exerted by a slate of new GM directors led by Whitacre and selected by the automaker's majority shareholder, the US Treasury.March appointmentHenderson, 51, became CEO in March after his predecessor, Rick Wagoner, was forced out by the Obama administration as part of the US government-funded restructuring of GM."The board decided - and Fritz agreed - that given where we are, it was time to make some changes," GM spokesman Chris Preuss said at a hastily arranged news conference.Whitacre, a former AT&T chief executive, became chairman of GM in July as part of a new board vetted by the US Treasury and intended to safeguard the government's $50 billion investment in the automaker.The US government has a majority stake in GM, but the Obama administration has repeatedly said that it is leaving oversight of the company to Whitacre and the board."This decision was made by the board of directors alone. The administration was not involved in the decision," a White House spokeswoman said.Who's next?Whitacre, who became the public face of GM in its first ad campaign after bankruptcy, appeared briefly before reporters at GM's headquarters in Detroit but did not take questions on why the board had chosen to part ways with Henderson.Whitacre said Henderson, who helped GM through its July bankruptcy, had "done a remarkable job in leading the company through an unprecedented period of challenge and change.""While momentum has been building over the past several months, all involved agree that changes needed to be made," Whitacre said.Whitacre, a plain-spoken Texan who said he knew nothing about the auto industry when he became GM chairman, has surprised GM insiders by making unannounced plant visits and putting blunt questions to workers at all levels.With his move to become GM's interim CEO, all three US automakers are now headed by outsiders to Detroit.Ford Motor Co CEO Alan Mulally left Boeing Co in 2006. Chrysler is now run by Fiat SpA CEO Sergio Marchionne.Whitacre has complained that pay restrictions imposed on GM have made it hard to hire external talent, but the board will likely look for an outsider who can better sell the vision of a revived GM, analysts and industry executives said."Whitacre wants an outside CEO. He's looking for another Alan Mulally," said Logan Robinson, a longtime auto industry executive and professor at the University of Detroit Mercy School of Law.Efforts to sell brandsHenderson, a career GM executive, had vowed when he became CEO to reform the slow-moving culture that contributed to the automaker's collapse.But GM's faltering efforts to sell off its laggard brands dominated Henderson's short tenure and tarnished his reputation as a dealmaker and raised questions about the company's strategy.First, Detroit-based dealership group Penske Automotive Group pulled out of a deal that would have had it acquire GM's Saturn brand in late September.Then last month, GM's board shifted course on a planned sale of the company's European Opel unit, rejecting a deal that Henderson had backed and helped broker.In the most recent setback, Swedish luxury car builder Koenigsegg dropped a planned acquisition of GM's Saab brand.Last month, in statements that some read as an implicit rebuke of Henderson, Whitacre seemed to question the aggressive timetable for an initial public offering of GM stock that had been outlined by Henderson."At the end of the day, it seems a little bit inevitable," said David Bitterman, managing director at Huron Consulting. "Obviously, Whitacre and Fritz didn't envision the new GM in the same way."GM's board said on Tuesday it would consider other potential deals to sell Saab over the next month but wind down its operations if it concluded that none could be reached.