Century City’s PacWest Bancorp announced Monday that it has agreed to buy Square 1 Financial Inc. in Durham, N.C., which specializes in lending to venture capital-backed companies, in an all-stock deal valued at about $849 million.

With $19.8 billion in assets as of Dec. 31, the combined bank would be the fourth-largest in Los Angeles County.

“We are very excited to align ourselves with the Square 1 team,” PacWest Chief Executive Matt Wagner said in a press release announcing the deal. “Thanks to the talented professionals and founders that built this successful institution, we are confident this transaction with Square 1 represents an excellent opportunity to grow core deposits, expand our nationwide lending platform and increase our presence in the dynamic technology and life-sciences markets.”

Shareholders of Square 1, which went public just last year, will receive 0.6 PacWest shares for each share of Square 1. The bank, which has about $3.1 billion in assets, operates one branch in North Carolina. PacWest, at more than $16 billion in assets, has 80 storefronts.

The Square 1 deal is just the latest in a string of acquisitions PacWest has made in recent years. Last April, the bank acquired downtown L.A. lender CapitalSource for $3.1 billion in cash and stock, a deal that vaulted it to L.A.’s fourth-largest bank. And prior to that, it picked up the assets of a handful of smaller regional banks that failed during the economic downturn.

The announcement came before markets opened Monday. PacWest shares closed the day at $44.99, down about 2 percent.