Clathrin-independent endocytosis: a unique platform for cell signaling and PM remodeling

Abstract

There is increasing interest in endocytosis that occurs independently of clathrin coats and the fates of membrane proteins internalized by this mechanism. The appearance of clathrin-independent endocytic and membrane recycling pathways seems to vary with different cell types and cargo molecules. In this review we focus on studies that have been performed using HeLa and COS cells as model systems for understanding this membrane trafficking system. These endosomal membranes contain signaling molecules including H-Ras, Rac1, Arf6 and Rab proteins, and a lipid environment rich in cholesterol and PIP(2) providing a unique platform for cell signaling. Furthermore, activation of some of these signaling molecules (H-Ras, Rac and Arf6) can switch the constitutive form of clathrin-independent endocytosis into a stimulated one, associated with PM ruffling and macropinocytosis.

Figure 1

Schematic illustration of CIE and CDE pathways showing lipid content and regulatory molecules. Constitutive CIE vesicles contain cargo such as MHCI (blue bars) while CDE vesicles contain cargo such as transferrin receptor (green bars). CIE vesicles lose PIP₂ and may acquire Rab5 prior to fusion with Rab5, EEA1-positive, “classical” early endosome (EE) containing CDE cargo. From here, CIE cargo may go on to late endosomes (LE) and lysosomes for degradation. CIE cargo destined for recycling reaches the endosomal recycling compartment (ERC) either directly or after fusion with “classical” EE and is recycled back out to the PM via tubular membranes that do not contain CDE cargo. Recycling back out to the PM requires Rab22 and probably Rab8 for tubule formation and Rab11, Arf6, Par 3, Par6, Cdc42 and actin for fusion back to the PM. Activation of Arf6, Ras and Src leads to PM ruffling and macropinocytosis that alters the architecture of the CIE. Macropinosomes mature from PIP₂/PIP₃-containing by loss of PIP₂, acquisition of Rab5 and loss of PIP₃. The bulk of the macropinosome membrane is recycled back out to the PM using CIE pathway recycling components.

Figure 2

Downstream effectors of Arf6, Ras and Src that contribute to PM ruffling, macropinocytosis, and membrane recycling. A partial list of effector proteins involved in macropinocytosis emphasizing overlapping targets. See text for details.