The entire board of South Africa’s state-owned state pension fund, the Public Investment Corporation (PIC), resigned on Friday, citing instability at the firm after graft allegations against at least four directors.

The PIC, which has around 2 trillion rand ($150 billion) of civil servants pensions under its custody and is Africa’s biggest pension fund, is facing a judicial inquiry on corporate governance after a small opposition party alleged that former chief executive Dan Matjila had misused funds and made careless investment decision.

The allegations against Matjila, a board member, were followed this week by an anonymous whistle blower who sent an email implicating three other board members including acting CEO Matshepo More as well as the board’s chairman and deputy finance Minister Mondli Gungubele in impropriety.

Matjila and Gungubele have denied any wrongdoing. More could not be reached for comment.

The board said the allegations were “unbearable to us as individuals” in a letter addressed to Finance Minister Tito Mboweni asking him to release them from their positions.

“Our assessment is that this may not be the end,” the letter signed by 11 directors said.

“There is clearly a concerted effort to discredit the board of directors to an extent that there cannot be any credibility to the work that is executed in fulfilling its fiduciary responsibilities.”

The spokesman for the Finance Ministry, which oversee the PIC, did not immediately respond to request for comment.