BOSTON, May 13 : Leaders of three of the biggest U.S. public pension systems said they have major concerns over SpaceX's "extreme" ownership and control set-up in its upcoming public stock listing, urging founder and CEO Elon Musk to remove provisions that would curb shareholder protections."We are writing to express our serious concerns with the reported novel and extreme governance structure and provisions SpaceX is planning to disclose in its registration statement," New York State Comptroller Thomas DiNapoli, New York City Comptroller Mark Levine and California Public Employees' Retirement System CEO Marcie Frost said in a letter sent Wednesday to Musk that was reviewed by Reuters.The officials - representing three of the top four largest public pension plans in the U.S. - objected to the amount of power the board has given Musk over the company, including voting control over the stock, veto power over his own removal as CEO, and protections from litigation, including mandatory arbitration for SpaceX shareholder claims.The SpaceX listing is expected to be the biggest initial public offering in history, with the company looking to raise $75 billion, with a $1.75 trillion valuation.