Nowadays, enterprise modeling has to concern with not only modeling of information systems but also modeling interoperability of them. This paper proposes an approach for modeling interoperability of information systems (IS). The approach considers that each information system is operated by a responsibility zone in the living world. Interoperability of information systems is based on the information overlapped between responsibility zones. The first step is to specify overlap situations between responsibility zones. The second step is to specify overlap protocols and the use of those overlap protocols to operate the overlap situations. The third step is to implement the overlap protocols using the current interoperability technologies.