Figure 8.

Docking, tethering, priming and fusion of lytic granules at the IS. Upon immune cell activation, synthesis of mature lytic granules occurs (step # 1). Several molecules, including LYST and the AP-3 complex, are involved in this process (not shown here). These two proteins are causative in the Chediak-Higashi syndrome and the type 2 Hermansky-Pudlak syndrome, respectively. Once mature lytic granules have reached the IS, sequential steps must take place to guarantee their proper docking, tethering, priming and fusion with the synaptic membrane. Before the arrival of lytic granules, Rab11a⁺ recycling endosomes fuse with the plasma membrane through a VAMP8-syntaxin 4 interaction (step # 2). Upon this fusion, syntaxin 11/STXBP2 complexes are deposited at the plasma membrane. After newly arrived lytic granules are docked at the IS (step # 3), tethering factors ensure that these granules remain firmly in place. This is mediated by several tethering proteins, such as Munc13-4 and Slp2a (step # 4). Simultaneously, Munc13-4 primes the lytic granules for the final fusion stage. Syntaxin 11/STXBP2 complexes, formerly brought by recycling endosomes, mediate the fusion of lytic granules with the synaptic membrane by interacting with VAMP7. After this fusion step, cytotoxic molecules present within the lytic granules are released within the synaptic cleft (step # 5). Note that the interfacial actin protrusions described in CD8⁺ T cells by Tamzalit et al. (215) are not shown.