May play a role in early endosome fusion upstream of RAB5, hence regulating receptor trafficking and fluid-phase transport. Enhances cellular sensitivity to TNF-induced apoptosis. Colocalizes with EEA1 and RAB5 at endosomal membrane fusion hot spots. May translocate to the endoplasmic reticulum in the early phase of apoptosis. The FYVE domain is important for binding to the endosomal membrane.