Since AA doesn't fly on its own planes to Berlin, you might be looking at a "codeshare" flight in which AA simply attaches its flight number to an Air Berlin flight. In that case the point is moot, since it will be Air Berlin's seats. And, since Air Berlin doesn't fly to Portland, you'd probably be looking at at least one segment on AA in order to get to an Air Berlin gateway city, of which there are only a few in North America.
With a business class award using AA miles, any domestic flights (e.g. Portland > Dallas) would book into first class on the short flights, business on the long haul. If your itinerary was via Air Berlin from Los Angeles (recently announced route) then it would be Alaska Airlines to LAX, since American doesn't fly from PDX to LAX.
American uses an "all partners" approach to award seats - you can use any number of AA partners (as well as AA itself) for award itineraries. To get from Portland to Berlin, you could fly on AA "metal" from PDX to Dallas, then AA business class to London, then British Airways to Berlin, or on Alaska/Horizon to Seattle, BA to London then Berlin, or numerous other permutations. Flights on British Airways transatlantic flights will incur hefty fuel surcharges (several hundred dollars each plus the miles) so be sure to ask about the bottom line when you speak to AA about the reservations.