Pathways and itinerary of clathrin-independent endocytosis

National Heart, Lung, And Blood Institute

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Abstract

This project is focused on understanding clathrin-independent forms of endocytosis (CIE). Endocytosis that occurs without clathrin coats occurs in all cells but is poorly understood. We are interested in studying the cargo proteins that enter cells by this mechanism, their intracellular itinerary once they have been internalized and whether they contain amino acid sequences that allow for specialized sorting within cells. We have been identifying new cargo proteins and found that a subset of these proteins take alternative traffic routes once they have entered cells. The major histocompatibility complex Class I protein (MHCI), is a prototypical clathrin-indepenent cargo protein and after internalization it reaches endosomes that contain cargo proteins such as the transferrin receptor that enter via clathrin-depenent endocytosis. From there, MHCI travels either to late endosomes and lysosomes where it is degraded or on to recycling tubules that return MHCI back to the cell surface. CD44, CD98, and CD147, however, show an altered itinerary in many cells where they traffic directly into the recycling tubules and avoid trafficking to lysosomes. Consistent with this altered itinerary, CD44, CD98 and CD147 are long-lived proteins and are not degraded like MHCI, which is routed to lysosomes. We have recently been looking to see whether specialized cell types also exhibit this sorting of cargo and we found that T cells and cells undergoing constitutive macropinocytosis also sort cargo in a manner similar to that observed in HeLa cells. We are interested in the role that ubiquitin modification of cargo proteins plays in regulating the sorting of cargo and routing of cargo to lysosomes for degradation. We previously showed that over expression of the MARCH8 E3 ubiquitin ligase leads to ubiquitination and degradation of CD98, normally a long-lived protein (Eyster et al 2011). We also published that expression of a de-ubiquitinase (DUB) or ubiquitin-specific protease (USP) can counteract the effect of expression of MARCH8. This DUB is called TRE17/USP6 and was previously shown to be unregulated in ABC bone cancer. Expression of TRE17 specifically de-ubiquitinates CD98 and returns its trafficking to the normal pathway (i.e. avoidance of lysosomes) (Funakoshi et al, 2014). However TRE17 did not rescue the effect of MARCH8 on degradation of the transferrin receptor. As a follow-up to this study, we are now analyzing the results of an siRNA screen of human de-ubiquitinases (DUBs), knocking down the 100 DUBs in the human genome. We hope to identify those DUBs that, in addition to TRE17 (USP6), are responsible for protecting CD98 and CD147 from lysosomal degradation. We obtained several hits and are currently validating these hits. Re-assuringly, TRE17/USP6 was indeed one of the hits that when depleted from HeLa cells led to reduced surface levels of CD98, reaffirming the observations made by Funakoshi et al.