The court, voting 5-4 along ideological lines, today said shareholders can’t sue Janus and a subsidiary for helping produce allegedly misleading prospectuses for Janus mutual funds. Janus contended that the funds are separate legal entities and that neither the parent company nor the subsidiary were responsible for the prospectuses.

The case was a follow-up to a 2008 Supreme Court decision that curbed securities suits against a company’s banks and business partners. Justice Clarence Thomas wrote the court’s majority opinion.

The mutual fund fight stemmed from allegations that Janus let preferred clients engage in market timing, a practice of making frequent, short-term trades at the expense of other investors.

The prospectuses said the funds had taken steps to deter market timing. In the lawsuit, Janus shareholders said those assurances were revealed to be false in 2003 when New York state officials filed a complaint against the company, causing its share price to fall. Denver-based Janus later agreed to pay $201 million and cut fees by $125 million to settle claims by state and federal regulators.

The shareholders were seeking to sue both Janus Capital Group and Janus Capital Management LLC, which serves as the investment adviser to the funds and has day-to-day responsibility for their management. The Obama administration backed the shareholders at the Supreme Court, arguing that the suit should be allowed to go forward.

The case is Janus Capital Group v. First Derivative Traders, 09-525.