Talk:Cell Import - Endocytosis

From CellBiology


Cytosolic Vesicles

  • Single membrane bound vesicles
  • two membrane processes involved in trafficking
    • budding
    • fusion
  • Linked to the ER and Golgi membrane system
    • Lysosomes
    • Peroxisomes

Membrane compartments

TEM Golgi Apparatus


  • organelles that metabolize fatty acids
  • increased activity of digestion in enzyme studies
    • de Duve C. The peroxisome: a new cytoplasmic organelle. Proc R Soc Lond B Biol Sci. 1969 Apr 15;173(30):71-83. PMID 4389648 (originally called microbodies)
  • Identified by EM 10 years later
  • enzymes that produce and others that degrade hydrogen peroxide (a reactive oxygen species, ROS)
    • oxidative reactions using molecular oxygen to generate hydrogen peroxide
    • oxidizing fatty acids, bile salts and cholesterol
    • then converting hydrogen peroxide to nontoxic forms
  • Catalases (EC
    • haem-containing proteins that catalyse conversion of hydrogen peroxide (H2O2) to water and molecular oxygen

Links: MBOC - EM Peroxisomes | MBOC - Peroxisomes | The Cell - Peroxisomes | Lippincott-Schwartz Lab | Electron micrograph of rat liver | MCB - synthesis of catalase

Peroxisome Assembly

  • Two theories on peroxisome formation
  • semiautonomous oranelles (like mitochondria) which multiply strictly by growth and division
    • Free ribosomes synthesize peroxisome proteins
    • Imported into pre-existing peroxisomes as completed polypeptide chains
    • Peroxisome growth from protein import
    • formation of new peroxisomes by division of old ones
  • other organelles such as ER role in formation and maintenance of peroxisomal membranes

Links: MBOC - A model for how new peroxisomes are produced | Lippincott-Schwartz Lab

Absorption Mechanisms

Endocytosis Types

Endocytosis Mechanisms

Receptor Mediated Endocytosis

  • General term for all mechanisms of absorbtion extracellular fluid and substances
  • substances bind to receptor sites
  • vesicle called endosome
  • can be utilized by viruses to enter cells


  • "cell drinking"
  • All cells, extracellular fluid
  • micropinocytosis within vesicles (<0.1 µm diameter)
  • macropinocytosis within vacuoles (0.5-5.0 µm) named macropinosomes

From: Protein Helps Orchestrate Cells' Fluid Uptake PLoS Biology Vol. 2, No. 9, e318 doi:10.1371/journal.pbio.0020318


  • “cell eating”
  • occurs only in specialized cells macrophages, dendritic cells and neutrophils
  • capture and destroy pathogens and particulate antigens
  • essential component of the innate immune response
  • Fc- and complement-receptor mediated phagocytosis, named for binding specificity for antibody tail region called Fc (Fragment, crystallizable)
  • clearance of apoptotic bodies
  • some bacteria "hijack" this process in non-phagocytic cells to enter and infect them

Links: NRG - The role of the endosomal–lysosomal apparatus | MBOC - Mechanisms used by bacteria to induce phagocytosis by nonphagocytic host cells |

Neutrophil chasing Bacteria and Phagocytosis

Page | Play
Remember this movie from the first lecture, where the white blood cell chases and phagocytoses a bacteria in a blood smear.

About the Movie: This video is taken from a 16mm movie made in the 1950s by the late David Rogers at Vanderbilt University. It was given to Dr. Tom Stossel via Dr. Viktor Najjar, Professor Emeritus at Tufts University Medical School and a former colleague of Rogers. It depicts a human polymorphonuclear leukocyte (neutrophil) on a blood film, crawling among red blood cells, notable for their dark color and principally spherical shape. The neutrophil is "chasing" Staphylococcus aureus microorganisms, added to the film. (Text from- Tom Stossel, June 22, 1999)


Invitrogen: human aortic smooth muscle cell - Actin-RFP Endosomes-GFP and Hoechst 33342

  • vesicle formed from plasma membrane budding
  • encloses extracellular fluid and substances
  • large ones called a phagosome or vacuole
  • pH 5.5 - recycling endosomes and intracellular secretory vesicles PMID 25385186

Clathrin-coated Pits

Clathrin-coated Pits

Clathrin triskelion Clathrin coat structure

  • Clathrin is a protein that coats both small membrane pits and coated vesicles
    • formed during endocytosis of materials at the surface of cells
  1. macromolecules to be internalized bind to specific cell surface receptors
  2. receptors are concentrated in regions of the plasma membrane (clathrin-coated pits)
  3. pits bud from the membrane to form small clathrin-coated vesicles containing the receptors and bound macromolecules

Mammalian endocytosis.jpg

Links: Imaging endocytic clathrin structures in living cells | comparing endocytosis in yeast and mammalian cells


  • lysosomal degradation pathway for cytoplasmic material
  • survival mechanism during short-term starvation
Autophagy (A) Upon induction of autophagy, a membrane sac called the isolation membrane (IM) forms and engulfs portions of the cytoplasm.

Sealing of its edges gives rise to the double-membrane bound autophagosome.

Fusion of the outer membrane with a lysosome results in formation of an autolysosome, in which the inner autophagosomal membrane and its contents are degraded.

PMID 16464128 | PLOS - Autophagy: A Forty-Year Search for a Missing Membrane Source

autophagy types

Links: MBOC - Image - Three pathways to degradation in lysosomes | eurekah - Image - EM Morphology of autophagosomes and autophagic vacuoles in isolated mouse hepatocytes | PLOS - Autophagy: A Forty-Year Search for a Missing Membrane Source


The Cell - Endocytosis and lysosome formation

  • Site of cellular digestion - contain up to 40 enzymes for digestion
  • Acid Hydrolases - active at acid pH (5)
  • Hydrogen ion pump in lysosomal membrane - drives ions from cytoplasm into lumenal space, generates internal acidic environment

Lysosome Membrane

  • Allows passage of uncharged molecules
  • Molecules enter, are charged and cannot leave

Lysosome Digestive Enzymes

  • Acid hydrolases - enzymes named on basis of substrate
    • Protease - digests proteins
    • Nuclease - digests neucleotides (DNA)
    • Glycosidase - digests carbohydrates (sugars)
    • Lipases - digests lipids (fats)
    • Phospholipases - digests phospholipids (membranes)
    • Phosphatases - removes a phosphate group

Lysosome Types

Lysosomes primary and secondary

Primary Lysosome
  • newly formed without digestive substrate
  • formed from budding Golgi apparatus
  • can be secreted by exocytosis

Primary lysosomes in neutrophils are called primary granules or A granules

Secondary Lysosome
  • active form enzyme + substrate
  • formed by vesicle fusion event

Endosomal-lysosomal apparatus.jpg

Links: NRG - The role of the endosomal - lysosomal apparatus

Lysosome-Associated Membrane Proteins

  • LAMP-1, LAMP-2
  • Knockout of LAMP-2 in mice lysosomal enzyme targeting, autophagy and lysosomal biogenesis


Ubiquitin proteasome system

The ubiquitin proteasome system is an important non-lysosomal protein degradation pathway.

  • a protein complex that degrades proteins by proteolysis
    • misfolded, unneeded or damaged proteins
  • not vesicles, proteins form a "stacked-ring" structure
  • proteins destined for destruction are ubiquitinated (ubiquitination)
    • attachment of one or more ubiquitin monomers to protein


Caveolae myoepithelial cells Caveolae

  • small membrane invaginations
  • defined by containing caveolin protein in the vesicle membrane
  • not always present in all cells
  • functions
    • lipid recycling
    • cellular signalling
    • endocytosis

Links: Parton RG, Simons K. The multiple faces of caveolae. Nat Rev Mol Cell Biol. 2007 Mar;8(3):185-94. Review. PMID: 17318224 FIGURE 3 Caveola endocytosis | Madame Curie Bioscience Database - Transmission electron micrographs of endothelial caveolae

Transport Vesicles

  • RER synthesized material is transferred by budding off of membrane
  • Forms transport vesicle
  • Transports substances to different cellular locations
  • Most transport to Golgi apparatus
  • Active microtubule-based transport
    • also may use microfilament transport

Endoplasmic Reticulum and Golgi

Endocytic Transgolgi Network (TGN)
  • Both these systems involved with both cell import and export
    • New proteins synthesized on membrane-bound ribosomes are transported through the Golgi apparatus
    • reach the trans-Golgi network (TGN) and sorted for delivery to various destinations
    • exocytosis and endocytosis pathway
  • The question is how these compartments "sort" components going in different directions?
  • biodigested products from the digestive lysosomal pathway need now to be delivered to the biosynthetic pathway
    • amino acids, nucleotides, carbohydrates, phospholipids, lipids, etc
By this stage you should now be able to fully label this figure

By this stage you should now be able to fully label this figure

Other Vesicles


  • nanometer-sized membrane vesicles invaginating from multivesicular bodies and secreted from different cell types
  • Function suggested as the eradication of obsolete proteins, antigen presentation, or "Trojan horses" for viruses or prions. (PMID: 16809645)

Endosomal Multivesicular Bodies (MVBs)/endosomes

  • a stage in endosomal development
  • A type of cytoplasmic vesicle (200 - 500 nmdiameter) that occurs when part of an endosome membrane invaginates and buds into its own lumen forming smaller contained vesicles.
  • smaller contained vesicles are degraded when the endosome fuses with a lysosome.
  • allows delivery of transmembrane proteins into the lumen of the lysosome for degradation.
  • compartments for receptor downregulation and as intermediates in the formation of secretory lysosomes. (PMID: 12892785)
  • delivery of transmembrane proteins into the lumen of the lysosome for degradation is mediated by the multivesicular body pathway. (PMID: 15569240)
    • The ESCRT (ESCRT-I, -II and -III) complexes form a network that recruits monoubiquitinated proteins and drives their internalization into lumenal vesicles within a type of endosome known as a multivesicular body. (PMID: 16689637)
  • essential for both sorting and multivesicular endosomes formation (PMID: 12892785)

Links: Biogenesis and function of multivesicular bodies. Piper RC, Katzmann DJ. Annu Rev Cell Dev Biol. 2007;23:519-47. Review. PMID: 17506697


  • Macropinocytosis defines a series of events initiated by extensive plasma membrane reorganization or ruffling to form an external macropinocytic structure that is then enclosed and internalized. The process is constitutive in some organisms and cell types but in others it is only pronounced after growth factor stimulation. Internalized macropinosomes share many features with phagosomes and both are distinguished from other forms of pinocytic vesicles by their large size, morphological heterogeneity and lack of coat structures. (PMID: 17760832)


  • fusion of endoplasmic reticulum (ER) with macrophage plasmalemma, underneath phagocytic cups, is a source of membrane for phagosome formation in macrophages (PMID: 12151002)
  • phagocytic cup
    • actin-based membrane structure formed at the plasma membranes
    • impaired in Wiskott-Aldrich syndrome (WAS)

Synaptic Vesicles

  • secreted vesicle
  • neuron specific
  • filled with neurotransmitter


  • membrane vesicle enclosing melanin
  • melanin is a light-absorbing pigment
  • skin melanocytes and retinal pigment epithelium cells
  • melanophages are cells that engulfed the released melanosomes (eg skin keratinocytes)
    • skin colour due to melanocytes level of activity not to the number of melanocytes


Lysosomal Disorders

  • Several inherited disorders of lysosomal metabolism
    • tested at birth by Gutherie heel-prick
  • OMIM Database 112 entries
  • Lack of a specific enzyme
  • Can isolate and measure enzyme activities

Danon disease

Mucopolysaccharidosis (MPS) IIIB (Sanfilippo Syndrome type B)

  • children develop disturbances of sleep, activity levels, coordination, vision, hearing, and mental functioning culminating in early death
  • deficiency in lysosomal enzyme N-acetyl-glucosaminidase (Naglu)

Peroxisome biogenesis disorders

(PBDs) Zellweger syndrome and neonatal adrenoleukodystrophy are fatal genetic diseases that are autosomal recessive.

Genes and Diseases

  • Genes and Disease selected range of human disorders by system.
  • OMIM Online Mendelian Inheritance in Man, an online database of genetic disorders and genes.


Below are some example historical research finding related to endocytosis from the JCB Archive.

1956 Catching sight of lysosomes Lysosomes are identified by Christian deDuve when a membrane barrier gradually dissolves, thus yielding the tell-tale release of an enzyme activity over time.

1964 Coated pits bring in the yolk A study of yolk protein uptake leads Thomas Roth and Keith Porter to propose that endocytosis is specific to a particular cargo and that the vesicle coat might be functioning in both selection and mechanical molding.

1967 How to make a lysosome Daniel Friend and Marilyn Farquhar find that transport pathways intersect: synthesized enzyme meets endocytosed protein in the lysosome.

1978 Viruses catch an endocytic ride into the cell Ari Helenius puts together snapshots of virus entry to form a coherent sequence of events.

Lecture Audio

The University has a system for automated recording of lectures called Lectopia. Lectopia requires login using your student number and unipass. I will be adding the link to each iLecture Audio following the Lecture. Due to the automated recording method, most lectures begin 4-5 minutes into MP3 recordings and occasionally stop before the lecture does.

Archive: 2013 | 2012 | 2011 Print Version | 2010 | 2009 | 2008 | 2006

pHuji, a pH-sensitive red fluorescent protein for imaging of exo- and endocytosis

J Cell Biol. 2014 Nov 10;207(3):419-32. doi: 10.1083/jcb.201404107.

Shen Y1, Rosendale M2, Campbell RE3, Perrais D4.


Fluorescent proteins with pH-sensitive fluorescence are valuable tools for the imaging of exocytosis and endocytosis. The Aequorea green fluorescent protein mutant superecliptic pHluorin (SEP) is particularly well suited to these applications. Here we describe pHuji, a red fluorescent protein with a pH sensitivity that approaches that of SEP, making it amenable for detection of single exocytosis and endocytosis events. To demonstrate the utility of the pHuji plus SEP pair, we perform simultaneous two-color imaging of clathrin-mediated internalization of both the transferrin receptor and the β2 adrenergic receptor. These experiments reveal that the two receptors are differentially sorted at the time of endocytic vesicle formation. © 2014 Shen et al.

PMID 25385186

Regulation of peroxisome dynamics

Curr Opin Cell Biol. 2009 Feb;21(1):119-26. doi: 10.1016/ Epub 2009 Jan 31.

Smith JJ, Aitchison JD. Source Institute for Systems Biology, 1441 N 34th Street, Seattle, WA 98103, USA.


Peroxisomes are single-membraned organelles ubiquitous to eukaryotic cells that house metabolic reactions that generate and destroy harmful oxidative intermediates. They are dynamic structures whose morphology, abundance, composition, and function depend on the cell type and environment. Perhaps due to the potentially damaging and protective metabolic roles of peroxisomes and their dynamic presence in the cell, peroxisome biogenesis is emerging as a process that involves complex underlying mechanisms of regulated formation and maintenance. There are roughly 30 known peroxins, proteins involved in peroxisome biogenesis, many of which have been conserved from yeast to mammals. This review focuses on the biogenesis of peroxisomes with an emphasis on the regulation of peroxisome formation and the import of peroxisomal matrix proteins in the model organism Saccharomyces cerevisiae. PMID 19188056

Clathrin couture: fashioning distinctive membrane coats at the cell surface

PLoS Biol. 2009 Sep;7(9):e1000192. Epub 2009 Sep 8.

Traub LM.

Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA. Comment on:

PLoS Biol. 2009 Sep;7(9):e1000191. PMID 19809570

Membrane organization and dynamics in cell polarity

Cold Spring Harb Perspect Biol. 2009 Nov;1(5):a001321.

Orlando K, Guo W.

Department of Biology, University of Pennsylvania, Philadelphia, 19104-6018, USA. Abstract The establishment and maintenance of cell polarity is important to a wide range of biological processes ranging from chemotaxis to embryogenesis. An essential feature of cell polarity is the asymmetric organization of proteins and lipids in the plasma membrane. In this article, we discuss how polarity regulators such as small GTP-binding proteins and phospholipids spatially and kinetically control vesicular trafficking and membrane organization. Conversely, we discuss how membrane trafficking contributes to cell polarization through delivery of polarity determinants and regulators to the plasma membrane.

PMID: 20066116