Thread mushrooms on 4 (8-inch) metal skewers; set aside. Combine soy sauce, margarine, and garlic powder. Brush half of soy sauce mixture on mushrooms.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place kabobs on rack; grill, covered, 4 to 5 minutes or until mushrooms are tender, turning and basting occasionally with remaining soy sauce mixture.