In a major shake-up of the executive suite, Fannie Mae chief executive Michael Williams announced his resignation this week, effective as soon as the company’s board chooses a successor.Williams’ resignation follows last October’s announcement by Freddie Mac CEO Charles Haldeman that he would step down from the company sometime in 2012.Williams spent 21 years at Fannie in a variety of capacities, most notably as the executive responsible for overseeing the company’s financial restatements, and accounting and control reforms pre-conservatorship and as chief operating officer. In April 2009, he was named CEO.