Client–Server P Systems in Modeling Molecular Interaction Gabriel Ciobanu 1 , Daniel Dumitriu 2 , Dorin Huzum 2 , Gabriel Moruz 2 , and Bogdan Tanas˘ a 3 1 National University of Singapore, School of Computing gabriel@comp.nus.edu.sg 2 “A.I.Cuza” University of Ia¸ si, Romania {danield,huzzy,gabbym}@infoiasi.ro 3 Institute of Computer Science, Romanian Academy, Ia¸ si bogdan@iit.tuiasi.ro Abstract. We present a new version of P systems called Client–Server P Systems (CSPS). The client membranes are characterized by their states; the server membrane stores the states of the clients and trig- gers the corresponding interaction rules. We show that CSPS have the same expressive power as Turing machines. CSPS is used to model var- ious molecular processes in which interaction and state transitions are causally linked. Signaling pathways and T cell activation are described by using a CSPS software environment called MOlNET (MOlecular NET- works). MOlNET can describe the dynamics of molecular interactions, including both qualitative and quantitative aspects and simulating the signaling pathways that tune the activation thresholds for T cells. 1 Introduction Membrane computing is based on membrane systems or P systems, a new class of distributed and parallel computing devices introduced in [12]. The approach is based on hierarchical systems: finite cell-structures consisting of cell-membranes embedded in a main membrane called the skin. The membranes determine re- gions where objects, elements of a finite set, and evolution rules can be placed. The objects evolve according to given rules associated with a region. Objects may also move between regions. A computation starts from an initial configu- ration of the system, and terminates when no further rule can be applied. A software simulator of membrane systems is presented in [3]. Hierarchical systems are well-known structures in computer science, and the notion of computation based on evolution rules is common. The interpretation of the computation is rather new: the result of a computation is a multiset of objects collected in the output cell or sent out of the system. The behaviour of the whole system is obtained by combining the resulting multisets, or consider- ing the multiplicity of objects present in a specific output membrane of a final configuration. G. P˘aun et al. (Eds.): WMC-CdeA 2002, LNCS 2597, pp. 203–218, 2003. c  Springer-Verlag Berlin Heidelberg 2003