WASHINGTON (AP) — Investors could lose principal from money market investment funds that underperform under rules being proposed by the Securities and Exchange Commission. But the change would affect mainly institutional rather than individual investors.

The SEC was to vote Wednesday to advance two plans that are intended to better inform investors and protect the industry from risks that surfaced at the height of the 2008 financial crisis.

One plan would allow shares of some money-market funds to "float", instead of having a fixed value of $1 per share. The proposal failed to gain support last year but has since won the backing of a panel of regulators that include Federal Reserve Chairman Ben Bernanke.

The other would impose fees on withdrawals from funds if their assets that can be readily converted into cash fall below a certain level.

Allowing values to float would make some money funds more like bonds, whose principal changes with increases or decreases in interest rates. That's a fundamental shift for the investments. But proponents say it is necessary because it would show money funds, while safer than stocks and many other investments, still carry some level of risk. They say more awareness of the risk would reduce the potential for runs on money funds.

The SEC proposal would limit the floating-value requirement to those money-market funds known as "prime."

Prime funds attract mainly big institutional investors as opposed to retail customers and are considered more risk-prone because they invest in short-term corporate debt. They represent roughly half of the total $2.9 trillion assets held by all money-market mutual funds.

The SEC will vote to open proposed changes to public comment, which is likely to last for several months. At some later point, the agency would finalize the rules or settle on a modified version of them.

Mary Schapiro, who stepped down as SEC chairman in December, pushed unsuccessfully last year for a floating value for all money-market funds and a requirement that money funds hold capital reserves of 1 percent of the fund's assets. But three of the five commissioners opposed those changes and her proposal was never brought to a vote.

Some commissioners appeared to be sympathetic to arguments made by representatives for the industry, who complained that most of the changes previously recommended would make money funds unattractive and lead to fewer investors.

This time, however, the SEC is under pressure from the Financial Stability Oversight Council, a group of high-level regulators that has backed both the floating-value requirement and calls for strict capital reserves. Bernanke and Treasury Secretary Jacob Lew both sit on the panel.

Investors learned how risky mutual funds could be during the financial crisis. The Reserve Primary Fund, one of the largest money market funds, lost so much money that it "broke the buck." As a result, its value fell to just 97 cents per share.

The decline stoked fears over the safety of money funds. In the ensuing week, investors pulled out around $300 billion from prime money funds, representing 14 percent of the assets in those funds. The government stepped in to temporarily guarantee assets of all money funds so investors could be assured they would be protected from losses.