We’ll see whether this slows the company’s plan to split into separate entertainment and sports entities. But there’s not much Madison Square Garden can do about that: CEO Tad Smith announced this morning that he’ll leave at the end of the month to take the top job at Sotheby’s auction house. MSG Chairman James Dolan, who also runs Cablevision, will steer his family-controlled operation while it looks for a new CEO.

Smith became CEO a year ago, after working for five years at Cablevision. While there he ran Local Media, which included ad sales, the News 12 Networks, and Newsday.

The move to the auction business “was a very difficult decision for me, but is an opportunity that I could not pass up,” Smith says.

He leaves at a time when MSG is going through an important transformation. Last month Dolan named Gregg Siebert — the former Merrill Lynch head of global media practice — to be Vice Chairman of MSG. That was widely seen as a precursor to a period of active dealmaking.

The company has been moving ahead with a spinoff plan announced in October. It will create a company that controls and manages entertainment at venues including Madison Square Garden, New York’s Beacon Theater and Radio City Music Hall, the Forum in suburban Los Angeles, the Chicago Theater and Boston’s Wang Theater. Another sports-focused company will have MSG’s sports franchises, including the New York Knicks and Rangers; regional sports services MSG Network and MSG +; and investments in SiTV Media, which owns NUVOtv and Fuse.