California Code, Financial Code - FIN § 1583

Any trust company or bank authorized to engage in the trust business holding trust funds awaiting investment or distribution may deposit or leave on deposit such funds with any state or national bank.  Such funds shall not, however, be deposited or left with the same corporation or association depositing or leaving on deposit such funds or with any corporation or association holding or owning a majority of the capital stock of the trust company or bank making or leaving such deposit, unless such corporation or association shall first pledge, as security for such deposit, securities eligible for investment by savings banks having a market value of not less than the funds so deposited.  No security shall be required with respect to any part of such deposits which is insured under the provisions of any law of the United States.