Provided is an internal structural mullion for a standing seam panel system. A two-piece assembly includes a man extrusion having one hooked portion and a secondary extrusion having an opposing hooked portion. A cavity is defined on the main extrusion, and the secondary extrusion includes a foot wherein upon insertion of the foot into the cavity, the foot forms a fulcrum about which the secondary extrusion can bend away from the main extrusion and be tightened or loosened by the tightening or loosing of a screw, as a result forming a clamp for engaging panels of the panel system with variable pressure. Further, when clamped a continuous gasket seal occurs at a level just inside the panels so as to form a barrier against the intrusion of water.