Morgan Stanley and Goldman Sachs are to become bank holding companies, which potentially means that they will soon accept customer deposits.
Both banks will also now be more directly regulated by the Federal Reserve.
The decision to re-shape their business models comes in the wake of extreme turbulence on the financial markets over recent days.
Lehman Brothers has been forced into bankruptcy after investor concerns over its liquidity led to sharp falls in its share price, while Merrill Lynch moved to merge with Bank of America.
The other member of Wall Street's "big five", Bear Stearns, collapsed in March and was bought by JPMorgan in a Fed-brokered deal.
Lloyd C Blankfein, chairman and chief executive of Goldman Sachs, said: "While accelerated by market sentiment, our decision to be regulated by the Federal Reserve is based on the recognition that such regulation provides its members with full prudential supervision and access to permanent liquidity and funding."
John Mack, head of Morgan Stanley, added: "This new bank holding structure will ensure that Morgan Stanley is in the strongest possible position - with the stability and flexibility to seize opportunities in the rapidly changing financial marketplace.
"It also offers the marketplace certainty about the strength of our financial position and our access to funding."