Modern ACLs, such as FIPA ACL, provide standardised catalogues of performatives and protocols, designed as general purpose languages to ensure interoperability among agent systems. However, recent work reports a need for new ad-hoc sets of performatives and protocols in certain contexts, showing that FIPA ACL does not support adequately all relevant types of interactions. In this paper we first present a formal model that relates performatives and protocols to the organisation of MAS. Then, a principled method for the design of the ACL of a particular MAS is developed, which accounts for both, reusability and expressiveness. Finally, we illustrate our approach by an example in the domain of agent-based decision support for bus fleet management.