Endocytic Sorting and Recycling Require Membrane Phosphatidylserine Asymmetry Maintained by TAT-1/CHAT-1

Baohui Chen,Sheng Zeng,Wei Zou,Yan Zhang,Xin Li,Jiacong Yan,Yue Jiang,Xiaochen Wang,Andrew D Chisholm

doi:10.1371/journal.pgen.1001235

Abstract

Endocytic sorting is achieved through the formation of morphologically and functionally distinct sub-domains within early endosomes. Cargoes destined for recycling are sorted to and transported through newly-formed tubular membranes, but the processes that regulate membrane tubulation are poorly understood. Here, we identified a novel Caenorhabditis elegans Cdc50 family protein, CHAT-1, which acts as the chaperone of the TAT-1 P4-ATPase to regulate membrane phosphatidylserine (PS) asymmetry and endocytic transport. In chat-1 and tat-1 mutants, the endocytic sorting process is disrupted, leading to defects in both cargo recycling and degradation. TAT-1 and CHAT-1 colocalize to the tubular domain of the early endosome, the tubular endocytic recycling compartment (ERC), and the recycling endosome where PS is enriched on the cytosolic surface. Loss of tat-1 and chat-1 function disrupts membrane PS asymmetry and abrogates the tubular membrane structure. Our data suggest that CHAT-1 and TAT-1 maintain membrane phosphatidylserine asymmetry, thus promoting membrane tubulation and regulating endocytic sorting and recycling.

Highlights

In eukaryotic cells, internalized cargoes are transported to early endosomes where they are sorted to be recycled back to the plasma membrane, degraded in lysosomes or delivered to the trans-Golgi network
We show that two proteins, CHAT-1 and TAT-1, regulate membrane structure and are important for normal endocytic transport in the nematode worm C. elegans
Loss of tat-1 and chat-1 function disrupts the asymmetric distribution of PS, abolishes tubular membrane structures, and abrogates endocytic sorting/ recycling

Summary

Introduction

In eukaryotic cells, internalized cargoes are transported to early endosomes where they are sorted to be recycled back to the plasma membrane, degraded in lysosomes or delivered to the trans-Golgi network. The recycling vesicles which arise from the tubular extensions may undergo fast recycling, by fusing directly with plasma membranes, or slow recycling, by transporting cargoes through the endocytic recycling compartment (ERC), a collection of tubular membrane structures arranged around the microtubule-organizing center [7,8]. Both cargo sorting and subsequent recycling require extensive membrane remodeling to form tubular extensions, which have a high ratio of surface area to luminal volume, thereby effectively concentrating cargoes on recycling membranes. It is not clear at present how these tubular processes are formed and maintained

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Conclusion