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The N-Terminal Carbohydrate Recognition Site of the CationIndependent Mannose 6-phosphate Receptor: The Internal Bouncer
St Joan Antida SMART Team: Ashley Miller, Affnan Mohammad, and Mary Sayles
Advisor: Mary Carlson
Mentor: Nancy Dahms, Ph.D., Medical College of Wisconsin
Abstract:
In order for the human body to function properly,
lysosomes are necessary. Lysosomes are found in
virtually every cell in the body, and they rid cells of
metabolic waste through the process of hydrolysis.
Important assistants to the formation of lysosomes are
mannose 6-phosphate receptors, which guide the
hydrolytic enzymes, to the lysosomes. These hydrolytic
enzymes are tagged with a phosphorylated carbohydrate,
mannose 6-phosphate, which allows for the recognition by
the mannose 6-phosphate receptors. The hydrolytic
enzymes are responsible for breaking down the metabolic
waste within the lysosomes and are therefore essential to
be taken into the lysosome. Without the mannose 6phosphate tag, the receptors cannot recognize the
enzymes, thus denying the delivery of the enzymes into
lysosomes. The receptor has three sites to which the
mannose 6-phosphate binds. The 300-kDa receptor plays
a vital role in trafficking newly synthesized mannose 6phosphate containing acid hydrolases to the lysosomes. It
is not yet understood how the three sites are able to
interact with one another. Though rare, lysosomes can
malfunction, causing a toxic buildup, which damages and
eventually kills the cell. In this case, Lysosomal Storage
Disorder (LSD) results.
Tay Sachs
-Tay-Sachs is a genetic neurological disease and can
occur only if both parents are carriers of the mutated TaySachs gene.
Functions of Lysosomes:
-By taking in enzymes,
lysosomes can dispose of
Abnormal Proteins
-Release of endocytosed
nutrients, such as cholesterol
from LDL
Figure 1: A normal cell containing a
lysosome (yellow).
Lysosome
Figure 2: A lysosome with a
-Inactivation of pathogenic
organisms such as bacterium
mannose-6-phosphate receptor.
Function of MannoseMannose-6-Phosphate Receptor:
The Mannose-6 Phosphate Receptor acts as an “internal
bouncer” by deciding what can go into the lysosome and
what cannot. The receptor recognizes the Mannose-6
phosphate (M6P) tag found on approaching enzymes, as
shown in Figure 2. The M6P receptor denies entry of all
other enzymes that have do not have a M6P tag on them,
such as the enzyme shown in Figure 2 that has an Nacetyl-glucosamine (NAG) tag.
-Cellular Survival: partial
digestion of intracellular
organelles such as
mitochondrion occurs during
starvation
Lysosomal Storage Disorders (LSD)
-Mannose 6-phosphate receptors are essential for the entry of
enzymes into the lysosomes.
-A lysosome is a cell organelle that contains enzymes that
digest particles and disintegrate the cell after its death.
“Multi-domain” binding site:
-A Lysosomal Storage Disorder is a genetic defect, which
leads to an increase of substrates in the cell lysosomes.
1
M6P
-LSD’s have been around since the 1800’s.
2
-Tay Sachs is the most well known LSD.
4
M6P-GlcNAc
5
-The symptoms: Heightened startle reflex, a "cherry-red"
spot found on the retinas of the eyes,
and failure to develop motor skills.
-As time progresses, the child stops
behaving normally and death usually
occurs by age five.
-Currently, there is neither a cure nor effective treatment.
2
6
7
8
P
1
M6P
9
10
2
3
CI-MPR (Cation-Independent
Mannose 6-Phosphate
Receptor) has 3 distinct M6P
binding sites that interact with
lysosomal enzymes
P
11
3
12
13
14
15
- The damaging effects of Tay-Sachs begin when the child
is still in the womb.
1
3
P
1
-There is a certain enzyme, which clears out the fatty
protein found in cells. The fatty protein, GM2 ganglioside,
builds up in brain cells and eventually killing these cells.
domains 1 + 2 enhance
binding affinity by
domain 3 ~1000-fold
P
Cytosol
= ~150 amino acids
300kDa CI-MPR
Figure 3:
3 -The extracellular region of the CI-MPR contains 15
domains. The receptor can bind lysosomal enzymes at three
different sites, domains 1-3, domain 5, and domain 9.
-The N-terminal binding site (domains 1-3), which acts as an
internal bounder deciding what goes in and what must stay out,
differs from the other binding sites in that three domains must
work together as a unit to form a high affinity binding site.
-To understand how domain 1, 2 & 3 work together to form a high
affinity binding site, domains 1-3 was crystallized and its 3dimensional structure was determined by x-ray crystallography.
Supported by the National Institutes of Health (NIH) – National Center for Research Resources Science Education Partnership Award (NCRR-SEPA)