100g of fresh portabella mushrooms contain 26 calories, 0.2 grams fat, 5.1g carbs, 1.5g fiber, and 2.5g protein. Not much protein in there - mostly carbs and fiber.
Portobella mushrooms are about 25% protein, which is a good amount. (10-15% is what the American Dietetic Society recommends overall, and 20% is a good upper limit).
If you ever have questions on the nutrition content of foods, you can find most information here: http://www.nutritiondata.com/
Just type in the food you are looking for, and it will give you the nutritional breakdown.