Clathrin-independent pathways do not contribute significantly to endocytic flux

  1. Vassilis Bitsikas
  2. Ivan R Corrêa
  3. Benjamin J Nichols  Is a corresponding author
  1. MRC Laboratory of Molecular Biology, United Kingdom
  2. New England Biolabs, Inc., United States

Abstract

Several different endocytic pathways have been proposed to function in mammalian cells. Clathrin-coated pits are well defined, but the identity, mechanism and function of alternative pathways have been controversial. Here we apply universal chemical labelling of plasma membrane proteins to define all primary endocytic vesicles, and labelling of specific proteins with a reducible SNAP-tag substrate. These approaches provide high temporal resolution and stringent discrimination between surface-connected and intracellular membranes. We find that at least 95% of the earliest detectable endocytic vesicles arise from clathrin-coated pits. GPI-anchored proteins, candidate cargoes for alternate pathways, are also found to enter the cell predominantly via coated pits. Experiments employing a mutated clathrin adaptor reveal distinct mechanisms for sorting into coated pits, and thereby explain differential effects on the uptake of transferrin and GPI-anchored proteins. These data call for a revision of models for the activity and diversity of endocytic pathways in mammalian cells.

Article and author information

Author details

  1. Vassilis Bitsikas

    MRC Laboratory of Molecular Biology, Cambridge, United Kingdom
    Competing interests
    No competing interests declared.
  2. Ivan R Corrêa

    New England Biolabs, Inc., Ipswich, United States
    Competing interests
    Ivan R Corrêa, An employee of New England Biolabs Inc. New England Biolabs Inc. has a commercial interest in successful application of reagents used in this study.
  3. Benjamin J Nichols

    MRC Laboratory of Molecular Biology, Cambridge, United Kingdom
    For correspondence
    ben@mrc-lmb.cam.ac.uk
    Competing interests
    No competing interests declared.

Reviewing Editor

  1. Suzanne R Pfeffer, Stanford University, United States

Version history

  1. Received: July 11, 2014
  2. Accepted: September 15, 2014
  3. Accepted Manuscript published: September 17, 2014 (version 1)
  4. Version of Record published: October 6, 2014 (version 2)

Copyright

© 2014, Bitsikas et al.

This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.

Metrics

  • 6,724
    views
  • 823
    downloads
  • 145
    citations

Views, downloads and citations are aggregated across all versions of this paper published by eLife.

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Vassilis Bitsikas
  2. Ivan R Corrêa
  3. Benjamin J Nichols
(2014)
Clathrin-independent pathways do not contribute significantly to endocytic flux
eLife 3:e03970.
https://doi.org/10.7554/eLife.03970

Share this article

https://doi.org/10.7554/eLife.03970

Further reading

    1. Biochemistry and Chemical Biology
    2. Cell Biology
    Christopher TA Lewis, Elise G Melhedegaard ... Julien Ochala
    Research Article

    Hibernation is a period of metabolic suppression utilized by many small and large mammal species to survive during winter periods. As the underlying cellular and molecular mechanisms remain incompletely understood, our study aimed to determine whether skeletal muscle myosin and its metabolic efficiency undergo alterations during hibernation to optimize energy utilization. We isolated muscle fibers from small hibernators, Ictidomys tridecemlineatus and Eliomys quercinus and larger hibernators, Ursus arctos and Ursus americanus. We then conducted loaded Mant-ATP chase experiments alongside X-ray diffraction to measure resting myosin dynamics and its ATP demand. In parallel, we performed multiple proteomics analyses. Our results showed a preservation of myosin structure in U. arctos and U. americanus during hibernation, whilst in I. tridecemlineatus and E. quercinus, changes in myosin metabolic states during torpor unexpectedly led to higher levels in energy expenditure of type II, fast-twitch muscle fibers at ambient lab temperatures (20 °C). Upon repeating loaded Mant-ATP chase experiments at 8 °C (near the body temperature of torpid animals), we found that myosin ATP consumption in type II muscle fibers was reduced by 77–107% during torpor compared to active periods. Additionally, we observed Myh2 hyper-phosphorylation during torpor in I. tridecemilineatus, which was predicted to stabilize the myosin molecule. This may act as a potential molecular mechanism mitigating myosin-associated increases in skeletal muscle energy expenditure during periods of torpor in response to cold exposure. Altogether, we demonstrate that resting myosin is altered in hibernating mammals, contributing to significant changes to the ATP consumption of skeletal muscle. Additionally, we observe that it is further altered in response to cold exposure and highlight myosin as a potentially contributor to skeletal muscle non-shivering thermogenesis.