Translation:Egg is a very important source of protein.
That is the same sentence Duo proposed. If you wanted to say: "egg is a source of protein very important" that would be the same. To clarify if "muito importante" is related to the egg, you could say: "ovo é muito importante como fonte de proteína" or "ovo, como fonte de pronteína, é muito importante"
You would write it just like that. For the other sentence you were asking about above, "Egg is a source of very important protein", you would need to quantify the word "protein" to indicate whether it's a source of one very important protein or many. So you could write "Egg is a source of a very important protein" or "Egg is a source of very important proteins".