How would you capitalize a word in a book/article title when that word has an en dash in the middle of it? For instance, should "protein-protein interaction" be capitalized as "Protein-protein Interaction" or "Protein-Protein Interaction"?
Capitalize each word that is a word on its own, as in "Protein–Protein Interaction". If the hyphen adds a prefix that doesn't stand on its own, like "Non-protein Elements", then capitalize the prefix. But I've seen it both ways.