Download Tay Sachs: A Protein Targeting Disease

Tay Sachs: A Protein Targeting Disease
Lecture Outline
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Protein Targeting & Disease
The Importance of Protein Targeting
Resident ER Protein Targeting Sequences
Lysosomal Enzyme Targeting
Secretory & Nuclear Proteins
Lysosomal Enzymes: From ER Through the Golgi
Lysosomal Enzymes: From Golgi to Lysosome
The CURL Endosome
Recycling Lysosomal Enzymes from The Extracellular Medium
Tay Sachs & Protein Targeting
Tay Sachs - A Disease by Any Other Name
I-Cell Disease: The Importance of Lysosomal Enzyme Phosphorylation
Lysosomal Enzyme Targeting: MPR vs Non-MPR Pathways
Protein Targeting & Disease
Different types of proteins are localized in different parts of the cell where they can carry out their specific
functions. Other proteins are secreted. If proteins end up in the wrong place or don’t get to the right place
this can lead to abnormal cell function and/or serious diseases. One means by which the location of proteins
is determined is through the process of protein targeting. The mis-sorting of rhodopsin, which should go to
the rods in the sensory retina, can result in retina pigmentosa and blindness. Mis-targeting of vasopressin
receptors so they don’t enter kidney epithelial cells causes nephrogenic diabetes insipidus, the inability of the
kidney to concentrate water. In this lecture we will focus on lysosomal enzyme targeting an event that is
mediated via specific targeting sequences. In a future lecture, we'll show how lysosomes and other vesicles
that contain specific molecules are targeted to specific locales in the cell.
The Importance of Protein Targeting
• Different proteins function in different cellular locales
• Some proteins function extracellularly
• The cell puts "address labels" on proteins so they get to the right place
• The address labels are often specific sequences of amino acids &/or phosphorylated sugars
• Only recently has it become evident that protein targeting can be the basis of serious diseases
Resident ER Protein Targeting Sequences
• Some proteins remain in Endoplasmic Reticulum
• Needed for functioning of ER
• KDEL targets proteins to ER: Lys-Asp-Glu-Leu
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Secretory & Nuclear Proteins
• Glycoproteins to be inserted into the membrane or secreted lack M6P (Secretion)
• Nuclear Proteins: Nuclear Localization Sequence (NLS) & other sequences
• Other targeting mechanisms also exist
The following diagram shows the targeting events and results for three types of proteins: ER resident
proteins, some secretory proteins and most lysosomal enzymes. Note that any ER resident proteins that
escape from the ER are immediately recycled back to it.
Lysosomal Enzymes: From ER Through the Golgi
Using the above figure, let's now look at the details of how most lysosomal enzymes are moved from the
endoplasmic reticulum through the Golgi:
• Lysosomal hydrolase is glycosylated in the endoplasmic reticulum
• Hydrolase is transported to cis-golgi
• Terminal mannose residue is phosphorylated in cis-golgi in a series of steps involving the enzymes
GlcNAc phosphotransferase and phosphodiesterase
• Enzyme travels from cis- to trans- face of golgi (vesicular transport)
• M6P-Receptor in trans-golgi network binds M-6-P domain of hydrolase sending enzyme to the lysosome
Lysosomal Enzymes: From Golgi to Lysosome
The following figure shows the continuing story in the life of a lysosomal enzyme, taking it from the Golgi
to the mature lysosome. The graphic also shows how the M6P-Receptor is recycled to the trans-Golgi to pick
up more lysosomal enzymes. This shuttle bus approach to moving things around in cells is a common theme.
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• Bud containing lysosomal enzyme bound to the M6P-R is coated with clathrin and "coated" vesicle is
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released from the trans-Golgi membrane
Vesicle uncoats then fuses with a CURL endosome
A pH change in CURL (Compartment for Unbinding of Receptor & Ligand) endosome unbinds Receptor
+ Ligand
Receptor recycles to trans-golgi or to the surface of the cell
PO4 is removed from enzyme
The dephosphorylated ligand (mature lysosomal enzyme) is packaged into a transport vesicle that fuses
with other vesicles to form the mature lysosome
The CURL Endosome
A study of the events in the CURL endosome reveals how a simple change in the environment, in this case
pH, around a protein can change its conformation and alter its binding activity.
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After the transport vesicle has delivered its contents to the CURL endosome, the lysosomal enzyme initially
remains bound to the M6P-Receptor. A pH drop within the CURL, driven by an ATP-dependent hydrogen
ion pump. In this acidic environment, a conformational change occurs in the proteins causing a dissociation
of the lysosomal enzyme from the M6P-R. The latter is recycled while the lysosomal enzyme is
dephosphorylated producing the mature lysosomal enzyme.
Recycling Lysosomal Enzymes from the Extracellular Medium
Lysosomal enzymes are taken up from the extracellular medium by receptor mediated endocytosis. Notice
the similarity in the events from the formation of clathrin coated vesicles at the trans-Golgi to the formation
of lysosomes. The formation of the clathrin coat and the events of uncoating and vesicle fusion are dealt with
in the lecture on Receptor Mediated Endocytosis. More insight into vesicular movements is also given in that
lecture.
Tay Sachs: A Disease of Protein Targeting
In the lecture on lysosomes we saw that Tay Sachs was due to a lack of a single enzyme: hexosaminidase A.
Tay Sachs disease is one of several classified as protein targeting diseases. Many are also classified in other
ways as well. The following picture and points below provide more insight into this disease by summarizing
the underlying basis of Tay Sachs.
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Mutation in alpha-subunit of Hexosaminidase A
Hexosaminidase A is not phosphorylated
Can't move from ER to Golgi
Thus, excluded from lysosomes
Absence of enzyme can't digest the glycosphingolipid, Ganglioside GM2
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Tay Sachs - A Disease by Any Other Name
• Thus, Tay Sachs is a Glycosphingolipid (GSL) Storage Disease because the absence of hexoseaminidase A
prevents the complete digestion of GM2
• It is also a disease resulting from incorrect protein trafficking
• It is also a lysosomal storage disease because hexosaminidase A doesn't get stored in the lysosome
I-Cell Disease: The Importance of Lysosomal Enzyme Phosphorylation
• A Lysosomal Storage Disease
• Genetic disorder in which many enzymes are absent from lysosomes
• Due to the absence of GlcNAc phosphotransferase which leads to the inability to phosphorylate the
mannose residue of lysososomal enzymes
• Unphosphorylated lysosomal enzymes are excluded from the lysosome
• Inability to digest components in digestive and autophagic vacuoles leads to the buildup of secondary
lysosomes that appear as dense "Inclusions" in patients with I-cell disease
Lysosomal Enzyme Targeting: MPR vs Non-MPR Pathways
MPR pathway
• Mannose-6-Phosphate (M6P) Receptor pathway
• M6P is terminal phosphorylated sugar that targets the majority of hydrolytic enzymes to the lysosome
• Receptors for M6P exist in selected membranes (e.g., transGolgi Network or TGN; cell membrane) to
ensure this occurs
Non-MPR pathway
• Pathways that use other sequences to target enzymes to lysosomes
• Some lysosomal enzymes lack M6P and are targeted in different ways
• The classic lysosomal enzyme Acid Phosphatase 1 enters lysosomes via a non-MPR pathway
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As detailed below, lysosomal enzymes such as Hexosaminidase A that possess the M-6-P sequence follow
the MPR pathway in which they bind to MPRs in the TGN. This targets them directly to lysosomes. While
there are many non-MPR pathways, acid phosphatase 1 follows one that has been actively studied. Instead of
M-6-P, this enzyme possesses a trans-membrane domain that allows the enzyme to be inserted into the cell
membrane after it leaves the TGN. In the cytoplasmic tail of acid phosphatase 1 is the targeting sequence
YRHV which directs receptor mediated endocytosis of the enzyme. Due to this sequence, once the enzyme is
in an early endosome it gets targeted to the lysosome.
©Copyright 1998-2009 Danton H. O'Day
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