Mich. Comp. Laws § 16.108

Sec. 8. (a) Heads of principal departments, commissions or boards, principal executive officers of departments, commissions and boards shall receive compensation prescribed by law. (b) Directors of departments, commissions, boards and directors of departments, boards and commissions transferred to a principal department shall not engage in any business, vocation or employment other than their office. Members of boards and commissions may so engage unless specifically prohibited by law.