It is definitely not protein. I tried carnivore diet for a while (had massive issues tolerating carbs lol), and the higher my protein intake was, more hungry I felt. Reducing protein and increasing fat also increased satiety.

Turns out, it is fats that produce satiety signals, and the effect seems to be by far the strongest with saturated fats, weaker with monounsaturated fats, while polyunsaturated fats actually induce hunger as strongly or even more strongly than carbohydrates do. The idea that "protein induces satiety" is a side effect of the fact that most (though not all) protein foods tend to be quite fatty.