To increase the utilization of side-streams from the food industry more investigations of multiple protein systems could be beneficial, complementary to the extensive studies available on single sourced protein isolates. Here, high moisture content protein-rich side streams were investigated in single screw extrusion. Vegetable protein mixtures comprising potato protein, oat protein and wheat bran were considered. Potato starch and potato fibers were used as structuring agents. Microstructurally, all compositions were dominated by protein aggregates. The structural agents enhanced the interconnected starch phase and/or additionally introduced fiber aggregates, with little apparent influence on the protein aggregates size and distribution. The moisture content was found to exert a dominant influence on the shear viscosity of the compositions. In addition, a saturation behavior of the power law parameters over 45% moisture content was apparent in the presence of the structural agents. An improved knowledge of industrial side-streams processability could increase their use in novel food products and exploited to create products with improved nutritional profiles.