First do note that the subject is plural.

Secondly, the use of 'protein' without any article may suggest it involves more than one particualr protein; it's hard to be sure, unless your wider context makes it clear — but at least it would pay to be aware of that possibility.

As you have rightly identified, 'comprise' in EN can indeed mean 'contient', 'est constitué de', and even 'comporte'. Without in any way being an expert in this subject field, my instinctive feeling would be that here it has the meaning 'are foremd from...'.

However, other more expert contributors may be able to confirm or not that suggestion!