It was my understanding that the risk to terrestrial life from alien life contamination was essentially zero because there’s no reason to think they would have evolved to be able to infect anything on earth.
There is no requirement for similarity or sameness. Even if there aren't any that I know of that violate this - it's not in the definition that prions require contact with either the same or extremely similar proteins to have any affect.
Obviously, the dangers of runaway self-replication makes even the smallest samples of primitive life potentially more deadly. And, while primitive life is far more likely in the universe at large, it is astronomically unlikely to reach our solar system undirected.