Section 13. Investment of public funds and public retirement system assets. (1) The legislature shall provide for a unified investment program for public funds and public retirement system assets and provide rules therefor, including supervision of investment of surplus funds of all counties, cities, towns, and other local governmental entities. Each fund forming a part of the unified investment program shall be separately identified. Except as provided in subsection (3), no public funds shall be invested in private corporate capital stock. The investment program shall be audited at least annually and a report thereof submitted to the governor and legislature.
     (2) The public school fund and the permanent funds of the Montana university system and all other state institutions of learning shall be safely and conservatively invested in:
     (a) Public securities of the state, its subdivisions, local government units, and districts within the state, or
     (b) Bonds of the United States or other securities fully guaranteed as to principal and interest by the United States, or
     (c) Such other safe investments bearing a fixed rate of interest as may be provided by law.
     (3) Investment of public retirement system assets shall be managed in a fiduciary capacity in the same manner that a prudent expert acting in a fiduciary capacity and familiar with the circumstances would use in the conduct of an enterprise of a similar character with similar aims. Public retirement system assets may be invested in private corporate capital stock.