Century City engineering and construction giant Aecom is close to signing a lease for more than 110,000 square feet at One California Plaza.

Downtown real estate sources said Aecom, headquartered at SunAmerica Center, 1999 Avenue of the Stars, since late 2014, would shift employees from its former headquarters, the north tower of City National Plaza, 515 S. Flower St., where the company takes about 108,000 square feet.

Representatives of Boston real estate investment firm Beacon Capital Partners, One Cal Plaza’s owner, and Aecom declined to comment on negotiations. Josh Wrobel and Tim Miller of Jones Lang LaSalle Inc., representing Beacon, also declined to comment.

Though the terms of the deal could not be determined, the average monthly asking rent at the 1 million-square-foot Cal Plaza is $2.14 a square foot, according to real estate data provider CoStar Group Inc., potentially valuing the deal at more than $2.8 million a year. First-quarter average monthly asking rents downtown were $3.32 a foot.

Other tenants in the 42-story tower, now 88 percent leased, include professional services firm Towers Watson & Co., and law firms Nixon Peabody and Morgan Lewis & Bockius.

Aecom acquired San Francisco’s URS Corp. in October 2014 in a $6 billion deal that made it one of the largest construction and engineering firms in the world. It also purchased smaller Indianapolis firm Hunt Construction Group last summer. The firm has nearly 100,000 employees worldwide, with more than 1,000 in Los Angeles.