The CEO of Best Buy Co. (NYSE: BBY), a struggling big-box retailer of consumer electronics that's in the middle of a top-to-bottom restructuring, has resigned, the company said Tuesday.

Brian Dunn, who has worked at Minneapolis-based Best Buy for 28 years including three as its chief executive, will be replaced on an interim basis by G. Mike Mikan, a member of the company's board of directors. A search committee of the board has been formed to find a permanent replacement for Dunn.

Best Buy shares were down 2.6 percent, or 58 cents, to $22.07 late Tuesday morning after rising to $23.17 earlier. Trading volume was heavy.

Mikan is a former executive vice president and chief financial officer at UnitedHealth Group Inc. (NYSE: UNH) and was CEO of Potum, a health care services affiliate of UnitedHealth.

Best Buy, which has about 1,100 stores in the U.S., announced in March that it would be closing 50 big-box outlets and cut 400 corporate positions to save $800 million over three years.