Membrane-trafficking sorting hubs: cooperation between PI4P and small GTPases at the trans-Golgi network

Abstract

Cell polarity in eukaryotes requires constant sorting, packaging and transport of membrane-bound cargo within the cell. These processes occur in two sorting hubs: the recycling endosome for incoming material and the trans-Golgi network for outgoing material. Phosphatidylinositol 3-phosphate and phosphatidylinositol 4-phosphate are enriched at the endocytic and exocytic sorting hubs, respectively, where they act together with small GTPases to recruit factors to segregate cargo and regulate carrier formation and transport. In this review, we summarize the current understanding of how these lipids and GTPases regulate membrane trafficking directly, emphasizing the recent discoveries of phosphatidylinositol 4-phosphate functions at the trans-Golgi network.

Figure 1

Intracellular distribution of phosphoinositides and small GTPases involved in membrane traffic. The localization of the different phosphoinositides and the transport steps that small GTPases regulate are shown in yeast (a) and mammalian (b) cells. EE, early endosome; LE/MVB, late endosome/multivesicular body; ER, endoplasmic reticulum.

Figure 2

The cell’s central sorting hubs and their effector proteins. (a) The generation of a recycling endosome and the different subdomains present. (1) PI 3-kinase, as well as the Rab5 GEF complex, is recruited by active Rab5 to a nascent endosome. Generation of both Rab5-GTP and PI3P recruits the tethering and fusion factor EEA1, promoting the fusion of incoming endocytic cargo. (2) Fusion, tubulation, and fission, as well as maturation of the membranes results in the sorting of cargo at the recycling endosome. (3) The SAND-1/Mon1-Ccz1 protein complex is required for maturation of the Rab5 domain into Rab7 by disrupting the Rab5 self-amplification loop and promoting the generation of a Rab7 feedback loop. Cargo recognition by specific adaptor proteins, such as Hrs, collects and sorts cargo destined for the late endosome/MVB. (4) Other factors, such as SARA, sort cargo into domains for recycling through the Rab4 or Rab11 pathways. (b) Involvement of PI4P at the TGN sorting hub in yeast (left) and mammals (right). (1) Arf1 and PI 4-kinase contribute to Golgi PI4P and recruitment of Rab11 to PI4P-rich domains. (2) Normal Golgi morphology and tubulation by GOLPH3 and the unconventional myosin MYO18a. (3) Recruitment of GGAs/AP1/clathrin for sorting of proteins destined for endosomes. (4) Involvement of the PI4P- and Arf1-binding proteins FAPP1 and FAPP2 in traffic to the plasma membranes. (5) Retention and retrieval of a subset of Golgi resident enzymes by Vps74p and COPI vesicles. (6) Recruitment of Gga2/AP1/clathrin for sorting of proteins destined for endosomes. (7) Recruitment and assembly of the exomer coat for sorting of specialized cargo into vesicles. (8) Recruitment and conformational regulation of the Rab-GEF protein Sec2p for constitutive secretion. (9) Transport of secretory membranes by the myosin V, Myo2p.

Figure I

Graph showing the recent increase in reports about PI4P. The number of publications was obtained from a Pubmed search of the phrase “phosphatidylinositol 4-phosphate functions” and subsequent manual validation. Although the numbers are clearly an underestimate, it shows an increasing trend in our knowledge of PI4P functions.

Figure II

Schematic representing the localization of various proteins involved in tip growth. EE, early endosome; ER, endoplasmic reticulum; PI-PLC, PI specific phospholipase C.