FIGURE 1.

The endolysosome pathway: Extracellular signaling molecules upon binding to cell surface receptors can be engulfed by endocytosis. These endocytosed vesicles can mature and differentiate into early endosomes (pH 5.5–6.0), late endosomes (pH 5.5–5.0), lysosomes (pH 5.0–4.5), and endolysosomes (a fusion process of lysosomes and late endosomes). Various marker substances can differentiate early from late endosomes including Rab4 (early endosomes), and Rab5 and Rab7 (late endosomes). Both early and late endosomes regulate recycling processes that return constituent molecules back to plasma membranes. Late endosomes can produce multi-vesicular bodies, which can fuse with lysosomes or can be released from cells in the form of extracellular vesicles (exosomes). Lysosomes regulate the degradation of extracellular materials in endolysosomes produced by fusions with late endosomes. Lysosomes can also fuse with autophagosomes to form autolysosomes; sites where extracellular and intracellular components are degraded. EL, endolysosomes; ER, endoplasmic reticulum; EE, early endosomes; LE, late-endosomes; MVBs, multi-vesicular bodies; AP, autophagosomes; Rab, ras-related protein 4, 5 and 7).