The party responsible for processing distributions from retirement plans and performing any required tax-withholding.
For qualified plans, the payer is the plan administrator or trustee. In some cases, trustees assigns the responsibility of payer to financial institutions.
IRC § 3405(e) , Temp. Treas. Reg. §35.3405-1T, IRS Instructions for filing form 1099-R
Additional Helpful Information
The plan administrator may pass on the responsibly of paying and withholding to another party, such as a custodian or other financial institution. Treas. Reg. §31.3405(c)-1, Q&A-4, -5.