Bacterial endocytic systems in plants and animals: Ca2+ as a common theme?

Intracellular interactions between bacteria and host cells are widespread in nature. In this review, the similarity between the infection processes of bacteria in plant and animal cells will be addressed. As paradigms, we selected the symbiosis between rhizobia and leguminous plants, and the survival of intracellular pathogenic bacteria in animal cells. The rhizobial symbiosis with leguminous plants is a model system for the study of plant-bacterium interactions. Through this interaction, the bacteria are released in a vacuole-like structure, called the symbiosome. The molecular processes, which lead to a functional symbiosome, are far from known. However, membrane fusion processes, and therefore also Ca2+, are crucial to establish this highly specialized organelle-like structure. A homologous system is the infection by certain bacterial pathogens of animal cells. These bacteria enter their host via phagocytosis and avoid the fusion with lysosomes, resulting in a membrane-bound vacuole in which the pathogens survive. The origin and maturation of this phagosome depends on Ca2+-signaling processes in the host cell and on proteins that regulate membrane fusion processes, such as SNAREs, Rab proteins, synaptotagmins and calmodulin. The aim of this review is to compare the endosymbiosis in leguminous plants with the surviving pathogens in animal host cells with a focus on Ca2+-signaling and membrane fusion-related processes. For both systems, the interaction starts with a bacterial entry of the host cell. It will be demonstrated that in both cases Ca2+ is a crucial second messenger. However, more emphasis will be put on the comparison of the later stages of infection, i.e., the formation of specialized bacteria-containing vacuoles. From structural, functional, and proteomic data, it is clear that phagosomes and symbiosomes are more related to each other than originally assumed. Proteins such as V-ATPases, calreticulin, phosphatidylinositol-3-kinase, Rab proteins, and SNAREs are present in both the phagosome and the symbiosome membrane, indicating that common cellular processes are used for building these intracellular organelles.