PacWest Bancorp in Los Angeles has agreed to buy Square 1 Financial in Durham, N.C.

The $16.2 billion-asset PacWest will pay about $849 million in stock for the $3.1 billion-asset Square 1 in a transaction that is expected to close in the fourth quarter.

"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," Matt Wagner, PacWest's chief executive, said in a press release Monday.

Square 1's lending operations will continue to do business under its own brand as a division of PacWest's bank. Square 1 has a branch in Durham and 12 loan-production offices throughout the country.

For PacWest, the deal is the latest in a string of aggressive acquisitions; the company bought CapitalSource for $2.3 billion last April. Square 1 is selling less than a year after going public.

Sandler O'Neill and Wachtell, Lipton, Rosen & Katz advised Square 1. Jefferies and Sullivan & Cromwell advised PacWest.

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