The California Public Employees Retirement System (CalPERS) announced Monday that its chief operating investment officer, Wylie Tollette, is leaving the pension fund.

CalPERS said in a statement that Matt Flynn, an investment director at the fund, will become interim COIO once Tollette leaves the $345 billion fund on January 5, 2018. The pension said it will begin searching for Tollette’s permanent replacement immediately.

Tollette, who worked at CalPERS since 2014, had served as a major force behind the scenes at the fund. As COIO, he led investment performance and risk analytics initiatives, in addition to working on policy, operations, compliance, and business planning in the CalPERS investment office, according to the fund’s website.

“The structure, reporting accuracy, and operational efficiency [Tollette] brought to the investment office helped save the fund millions of dollars and worked to ensure the long-term sustainability of the CalPERS fund,” said CalPERS chief executive officer Marcie Frost in the statement.

Tollette helped to launch CalPERS’ private-equity accounting and reporting system, which helps the firm to report fee information from its private-equity investments. He perviously worked for Franklin Templeton Investments for 19 years, most recently as its senior vice president for portfolio analysis and investment risk management, and is returning to the mutual fund firm.

Flynn, who is taking Tollette’s place as COIO, has worked for CalPERS since 2003. In his most recent role, Flynn managed CalPERS’ investment operations and trade support functions, in addition to chairing the fund’s pricing and valuation committee, according to CalPERS’ website.