Heterotypic Endosomal Interactions Drive Emergent Early Endosomal Maturations

Endosomes undergo a process of maturation as they progress through intracellular trafficking events. The most prominent model of early endosomal maturation involves a phosphoinositide driven gain or loss of specific proteins at the level of an individual endosome, but how this exchange is initiated is not understood. Here, we use Lattice Light Sheet imaging to track very early and early endosomes at the population level. We demonstrate that direct inter-endosomal contact drives the maturation from very early (APPL1 positive) to early (EEA1 positive) endosomes. Using fluorescence lifetime, we show that this endosomal interaction is underpinned by the asymmetric binding of EEA1 to early and very early endosomes through the C- and N-terminal, respectively. Thus, stochastic microtubule-mediated inter-endosomal interactions through EEA1 provides a mechanism to bring temporal and population level control to the process of endosome maturation. Our findings indicate that APPL1-to-EEA1 positive endosomal maturation is not a result of autonomous endosomal events but is driven via heterotypic EEA1-mediated endosomal interactions. These results support the intriguing proposal that the cell has evolved certain mechanisms to bound the variance of typically stochastic processes to ensure more deterministic trafficking outcomes.