In the merger deal, Imperial Bancorp—-one of the largest banks still headquartered in Southern California—-agreed to be acquired by Comerica Inc. in a transaction that will give Imperial shareholders 0.46 share of Comerica stock for each share of Imperial. Inglewood-based Imperial has $7.4 billion in assets and 15 branches in the West, 12 of which are in Southern California.

Comerica is the nation's 24th largest bank. It has $41 billion in assets, thanks in part to its focus on commercial lending.

The combined bank will be the fourth-largest in California and the 19th largest in the United States. Detroit-based Comerica has been aggressively expanding on the West Coast for more than a year, while Imperial's stock was hit by loan-loss worries in the spring but since has rebounded.