THE ROLE OF MEMBRANE DOMAINS IN PROTEIN AND LIPID SORTING DURING ENDOCYTIC TRAFFIC

Abstract

The lipid and protein composition of the plasma membrane (PM) must be tightly controlled to maintain cellular functionality, despite constant, rapid endocytosis. Because de novo synthesis of proteins and lipids is energetically costly, the cell depends on active recycling to return endocytosed membrane components back to the PM. For most proteins, the mechanisms and pathways of their PM retention remain unknown. The work presented here shows that association with ordered membrane microdomains is fully sufficient for PM recycling and that abrogation of raft partitioning leads to their degradation in lysosomes. These findings support a model wherein ordered membrane domains mediate PM recycling of membrane components from the endosomal system. The next step was to identify the pathways and molecular players responsible for raft-mediated recycling. Using orthogonal transmembrane protein probes for raft and non-raft domains, I identified and validated cellular machinery that act as trafficking mediators specific for recycling of raft-associated proteins to the PM. This raft-mediated pathway is not dependent on the classical recycling pathways defined by Rab4 and Rab11, but instead represents a novel route for PM recycling of raft-preferring cargo from late endosomes. I implicate Rab3 as a central regulator of this pathway and show that the Rab3 family is essential for PM homeostasis, as abrogation of all four members of the Rab3 family disrupts PM recycling of lipid raft associated proteins. The findings reveal a fundamental role for raft microdomains in endocytic sorting and recycling and support a novel role for Rab3 as a central regulator of a previously unrecognized mechanism for PM and endosome homeostasis