We propose a novel collaborative caching framework to support spatial query processing in Mobile Peer-to-Peer Networks (MP2PNs). To maximize cache sharing among clients, each client caches not only data objects but also parts of the index structure built on the spatial objects. Thus, we call the proposed method structure-embedded collaborative caching (SECC). By introducing a novel index structure called Signature Augment Tree (SAT), we address two crucial issues in SECC. First, we propose a cost-efficient collaborative query processing method in MP2PNs, including peer selection and result merge from multiple peers. Second, we develop a novel collaborative cache replacement policy which maximizes cache effectiveness by considering not only the peer itself but also its neighbors. We implement two SECC schemes, namely, the periodical and adaptive SAT-based schemes, with different SAT maintenance policies. Simulation results show that our SECC schemes significantly outperform other collaborative caching methods which are based on existing spatial caching schemes in a number of metrics, including traffic volume, query latency and power consumption.