Bag-of-Tasks (BoTs) applications are loosely coupled parallel applications whose concurrent entities are independent of each other. In this paper, we propose a novel architecture, which integrates distributed resources across heterogeneous Grid systems to support large-scale BoT applications. We present our integration solution and introduce how our prototype is implemented. Finally, we demonstrate our solution with a use case from the domain of drug discovery. It turns out that our proposal not only provides an adequate tool to support the development of BoT applications, but also is coherent and operationally reliable.