This paper proposes ISS-Mac, an efficient MAC protocol for infrastructure-supported low-power wireless sensor networks. A central base station (sink node) is responsible for wide-network synchronization and multi-hop TDMA slots allocation. Providing an accurate topology information to the base station requires a lot of signaling overhead which affects energy efficiency of the protocol. We propose a progressive topology construction scheme to minimize this overhead and enhance scalability. In addition, the protocol facilitates integration of new nodes into the network while operation. Performance evaluation of the protocol shows that it achieves high energy savings compared to the popular S-MAC and the Full Active CSMA protocols.