Maguire Properties Inc. announced Friday it had agreed to acquire a 10-building portfolio of office buildings from an affiliate of CommonWealth Partners LLC for $1.51 billion.

Maguire expects to close on the deal, funded through assumed debt and financing provided by Credit Suisse First Boston, in the first quarter.

The acquisition includes 5 million square feet of office space, including the 1 million-square-foot trophy 777 tower in downtown Los Angeles, and four parcels of land entitled for another 1.5 million square feet. It also enhances the L.A.-based real estate investment trust’s position as the largest owner of Class-A office property downtown.

“Our emphasis is to expand in high job growth and severely supply constrained areas which should give us higher returns,” Robert F. Maguire, the company’s chairman and co-chief executive, said in a statement. “We believe this acquisition should be very accretive to the company.”

Besides the 777 tower, two other properties in the portfolio in L.A. County are: 700 N. Central Ave. and 801 N. Brand Blvd. in Glendale.

Overall, the purchase adds 3 million square feet of office buildings in Maguire’s core Southern California market, with three buildings in San Diego County and one in Orange County. The company also picked up buildings in Phoenix, Denver and Austin, Texas.

All the buildings were owned through Los Angeles-based CommonWealth’s Fifth Street Properties Portfolio, a partnership with Rockefeller Group International Inc. and the California Public Employees Retirement System.