For half a century the five-story white cement and marble structure with its 12-foot high rooftop has been a landmark on the Boston skyline as well as of the automobile industry.

Fuller Cadillac, the first and one of the largest Cadillac dealerships is closing its doors on Wednesday, a victim of skyrocketing operating costs, high taxes and the flight of its customers to the suburbs.

"It's not a question of our selling less Cadillacs. We sold 1,500 cars last year, but it's not enough to meet our $35,000 overhead," company President Peter Fuller said. "The facilities are just too big. You don't get auto companies that are five stories high anymore.

"You have a practically impossible situation here and there's just no way to beat it without us getting out," he said. "We've suffered substantial losses in the last few years."

The lavish 265,000-square-foot building with its traces of gothic architecture was built in 1928 by Fuller's father, former Massachusetts governor Alvin T. Fuller. His father's fame derived from his decision not to commute the death sentences of convicted Italian anarchist immigrants Nicola Sacco and Bartolomeo Vanzetti.

The building was the fulfillment of a pledge to Cadillac's founder Henry Leland to "build the most beautiful Cadillac sales establishment in the country." Peter Fuller recalled.

The structure on Bonston's Commonwealth Avenue will be purchased by Boston University on July 2, 1979 for $3 million, university officials said.

Fuller names among its clients Boston Pops conductor Arthur Fiedler, Polaroid inventor Edwin Land, champion boxers Marvin Hagler and Joe Frazier, Red Sox stars Ted Williams and Luis Tiant, as well as the Kennedy family and other Massachusetts and national political giants, including the Cabots, Lodges, Lowells and Saltonstalls.

Former governor Fuller, who went into the Cadillac business in 1903, is credited as the first car dealer to sell automobiles on monthly installments ("to the Massachusetts governor Curtis Guild because he got his dividends quarterly"), and the first in the country to take used cars trade-ins toward the purchase of a new automobile.

Shortly after the turn of the century, Fuller Cadillac was known as the largest auto dealership in the world, selling Cadillacs, La Salles and Packards, Fuller said. His father also brought the first imported cars to Boston from France in 1900 -- the de Dion Voiturettes, which looked more like a bicycle than an automobile. At that time, the company was operating at the Motor Mart in Boston's Park Square.

Peter Fuller will maintain his automobile dealership, car leasing agency and service outlet in the suburbs of Boston where operating costs are lower than in most suburban areas nationwide.

"On George Washington's Birthday we sold 22 Cadillacs in Boston, but we sold 30 Oldsmobiles in our Watertown dealership," he said, "and the costs in Watertown just don't compare to what we have to spend here just to operate."

The real crunch came in 1965 when Cadillac ended its distributorship system, Fuller said. His dealership had been the largest distributor in New England.

Fuller says his decision to close the Boston dealership has brought a wave of letters "from the bygone days. It's been a very nostalgic thing. This building has really been some kind of landmark in Boston for a long, long time."