LONDON (Reuters) - Anglo-South African financial services firm Old Mutual (>> Old Mutual plc) said it has sold a 25 percent stake in its U.S. fund management arm (>> OM Asset Management PLC) to China's HNA (>> HNA Holding Group Co Ltd) for $446 million (358 million pounds), as part of its plan to split itself into four companies.

Old Mutual, which says regulatory change has made its business too complex to run in its current form, is aiming to break into four parts by the end of next year.

It has said it plans to dual-list its UK asset management and African emerging markets businesses in London and Johannesburg and reduce stakes in U.S. firm Old Mutual Asset Management (OMAM) and South Africa's Nedbank (>> Nedbank Group Ltd.).

It had already started cutting its stake in OMAM.

The sale to HNA Capital US, part of conglomerate HNA's financial services unit, cuts Old Mutual's remaining stake in OMAM (>> OM Asset Management PLC) to 26 percent, the firm said in a statement late on Saturday.

HNA has been building up stakes in a series of companies across Europe and the United States. It is now the third biggest shareholder in Deutsche Bank (>> Deutsche Bank AG), raising its stake last week to 4.76 percent.

The two-stage OMAM deal involves the sale to HNA of a 10 percent tranche at $15.30 per share and 15 percent tranche at $15.75 per share, at an approximate $1 premium to Friday's closing price.

Two HNA Capital US directors are expected to join OMAM's board, replacing Old Mutual directors, Old Mutual said.

(Reporting by Carolyn Cohn and Rachel Armstrong; Editing by Elaine Hardcastle)