Not sure what you mean by "responsible", but:
As a wing, my KEY is the tackle at the snap to determine whether it's a run or pass based on how they're blocking. On a run to my side I would be watching the blocking (including the tackle) up until the runner approached the LOS, then I would shift my attention to the runner and the action around him. On pass plays the wings are usually downfield a bit assisting the BJ with the short patterns, so the U takes on the tackles as well as his usual snapper/guards duties.
