Enforced by multiple agencies of the US government, notably the Federal Deposit Insurance Corporation.

http://www.fdic.gov/news/news/press/2010/pr10266.html

An amendment to the Dodd-Frank Act by Senator Susan Collins (R-ME) designed to equalize large bank and small bank holding company Tier 1 capital requirements. The Collins Amendment, originally drafted by the FDIC staff imposes, over time, the leverage and risk-based standards currently applicable to US insured depository institutions on US bank holding companies, including US intermediate holding companies of foreign banking organizations, thrift holding companies and systemically important nonbank financial companies.

One of the effects of the Collins Amendment is to eliminate trust preferred securities as an element of Tier 1 capital. Implementing regulations must be issued no later than 18 months from the bill's effective date. The Collins Amendment also directs the appropriate federal banking supervisors to develop capital requirements for all insured depository institutions, depository institution holding companies and systemically important nonbank financial companies to address systemically risky activities. The Collins Amendment echoes changes that have been proposed and scheduled for full implementation come 2019, by the Basel Committee on Banking Supervision in the Basel III agreement.

The Dodd-Frank Act implementation of the Collins Amendment (Nixon Peabody PDF) Harvard Law School Summary of Collins Amendment

All federally insured depository institutions and systemically important nonbank financial companies.