Proteoglycans and Integrins Flashcards
Proteoglycans in the ECM The study of proteoglycans dates back to the beginning of the 20th century with investigations of “chondromucoid” from cartilage and anticoagulant preparations from liver (heparin)
The proteoglycan molecules in connective tissue form a highly____, gel-like “___ ____” in which the fibrous ____are embedded
The polysaccharide gel resists ____ forces on the matrix while permitting the rapid ____ of nutrients, metabolites, and hormones between the___ and the tissue ___.
Proteoglycans consist of _____ (called _____) linked to a core ____
Note – Non-ECM proteoglycans are _______and can either ___ the plasma membrane (type I membrane proteins) or are linked by a ___ ___.
The study of proteoglycans dates back to the beginning of the 20th century with investigations of “chondromucoid” from cartilage and anticoagulant preparations from liver (heparin) The proteoglycan molecules in connective tissue form a highly hydrated, gel-like “ground substance” in which the fibrous proteins are embedded The polysaccharide gel resists compressive forces on the matrix while permitting the rapid diffusion of nutrients, metabolites, and hormones between the blood and the tissue cells Proteoglycans consist of polysaccharides called glycosaminoglycans (GAGs) linked to a core protein Note – Non-ECM proteoglycans are membrane-bound and can either span the plasma membrane (type I membrane proteins) or are linked by a lipid anchor.
Glycosaminoglycans
GAGs are _____polysaccharide chains composed of ____ disaccharide units
One of the two sugars in the repeating disaccharide is always an ___ sugar (____ or ____), which in most cases is ____.
The second sugar is usually a ____ ____ (____ or ____) or ____
Because there are___ or ____ groups on most of their sugars, GAGs are highly ____ charged (the most anionic molecules produced by animal cells)
Because of the negative charges, proteoglycans bind ____ charged ions and form _____ _____ with trapped water molecules, thereby creating a hydrated gel S
even main groups of GAGs are distinguished according to their ____, the type of ____ between the sugars, and the___ and ____ of ____ groups:
(1) ____, (2) ___ ___, (3) ___ ___, (4) ___, (5) ___ ____, (6) ___ __ __and (7) ___ ___ __
_____ is the only GAG that is not sulfated
GlycosaminoglycansGAGs are unbranched polysaccharide chains composed of repeating disaccharide units One of the two sugars in the repeating disaccharide is always an amino sugar (N-acetylglucosamine or N-acetylgalactosamine), which in most cases is sulfated The second sugar is usually a uronic acid (glucuronic or iduronic) or galactose Because there are sulfate or carboxyl groups on most of their sugars, GAGs are highly negatively charged (the most anionic molecules produced by animal cells) Because of the negative charges, proteoglycans bind positively charged ions and form H-bonds with trapped water molecules, thereby creating a hydrated gel Seven main groups of GAGs are distinguished according to their sugars, the type of linkage between the sugars, and the number and location of sulfate groups: (1) hyaluronan, (2) chondroitin sulfate, (3) dermatan sulfate, (4) heparin, (5) heparan sulfate, (6) Keratan sulfate I and (7) keratan sulfate II Hyaluronan is the only GAG that is not sulfated
Synthesis and structure of proteoglycans
Structure of proteoglycans resemble a ___ ___ with many GAG chains extending from a core proteins
The core protein is made on ___ ___ ribosomes and threaded into the____of the ER
Glycosylation occurs initially as the protein enters the ___ ____ and then in the ___
____ sugars act as precursors and add ___ ___to the protein and then to the growing end of the ____ chain
The majority of GAGs linked to protein to form a proteoglycan are attached via a _____ linker that consist of ___ ___ ___ ___ - ___ residue in the protein core
Hyaluronan does not occur _____ linked to proteoglycans, but instead interacts ____ with proteoglycans via hyaluronan-___ ____; other GAGs attach to the core protein ____.
Except ___ ____, all GAGs are attached to proteins via a ___ or____residue; keratan sulfate is attached to ____.
Two types of _______ are the enzymes responsible for addition of ___.
Sulfation occurs after ____ of ___.
After synthesis, proteoglycans are ____ from the cells
Structure of proteoglycans resemble a bottle brush with many GAG chains extending from a core proteins The core protein is made on membrane-bound ribosomes and threaded into the lumen of the ER Glycosylation occurs initially as the protein enters the ER lumen and then in the golgi UDP sugars act as precursors and add sugar units to the protein and then to the growing end of the carbohydrate chain The majority of GAGs linked to protein to form a proteoglycan are attached via a tetrasaccharide linker that consist of GlcA–Gal–Gal–Xyl–Ser residue in the protein core Hyaluronan does not occur covalently linked to proteoglycans, but instead interacts noncovalently with proteoglycans via hyaluronan-binding motifs; other GAGs attach to the core protein covalently. Except keratan sulfate, all GAGs are attached to proteins via a serine or threonine residue; keratan sulfate is attached to asparagine Two types of glycosyltransferases are the enzymes responsible for addition of sugars Sulfation occurs after addition of sugars After synthesis, proteoglycans are secreted from the cells
Proteoglycans vary in size
Proteoglycans can contain as much as ___% carbohydrate by weight, mostly in the form of long, ____ GAG chains, each typically about 80 sugars long
The proteoglycan aggrecan, which is a major component of cartilage, has a mass of about 3 × 106 daltons with over ____ GAG chains
Few proteoglycans are much smaller and have only ____ GAG chains; decorin, which is secreted by fibroblasts, has a__GAG chain.
Proteoglycans can contain as much as 95% carbohydrate by weight, mostly in the form of long, unbranched GAG chains, each typically about 80 sugars long The proteoglycan aggrecan, which is a major component of cartilage, has a mass of about 3 × 106 daltons with over 100 GAG chains Few proteoglycans are much smaller and have only 1–10 GAG chains; decorin, which is secreted by fibroblasts, has a single GAG chain.
Aggrecan
A number of _____ interact with ____ to form large complexes
A well-characterized example is aggrecan
Core protein of aggrecan (shown in red line) contains:
- ____-terminal domain that can bind ____ with the help of (____protein)
- a central domain to which ____ sulfates and ____ sulfates are attached
- a carboxy-terminal domain that contains a _-____ _____ domain
A number of proteoglycans interact with hyaluronan to form large complexes A well-characterized example is aggrecan Core protein of aggrecan (shown in red line) contains: amino-terminal domain that can bind hyaluronans with the help of (link protein) a central domain to which chondroitin sulfates and dermatan sulfates are attached a carboxy-terminal domain that contains a C-type lectin domain
General functions of ECM proteoglycans
Provide ____ space around and between cells
Their GAG chains can form gels of varying ___size and charge density and thereby serve as selective sieves to regulate the ___ of ____ and cells according to their ___ , ____, or both
Through interaction with collagen, proteoglycans aid in the ___ ____ of most tissues
Bind various ___ ___ molecules, such as certain protein __ ___, and enhance (by preventing their ____) or___their signaling activity
Proteoglycans bind and regulate the activities of, other types of secreted proteins, including ___ ____ (proteases) and ___ ____
Provide hydrated space around and between cells Their GAG chains can form gels of varying pore size and charge density and thereby serve as selective sieves to regulate the traffic of molecules and cells according to their size, charge, or both Through interaction with collagen, proteoglycans aid in the structural organization of most tissues Bind various secreted signal molecules, such as certain protein growth factors, and enhance (by preventing their proteolysis) or inhibit their signaling activity Proteoglycans bind and regulate the activities of, other types of secreted proteins, including proteolytic enzymes (proteases) and protease inhibitors
Specific functions proteoglycans
___: Cell ___
___: Formation of___ ___ ___
__: ___of cornea
__:____of cornea, Binds ___
____: Anticoagulant, cause release of ___ ____
___: Component of skin fibroblast and aortic wall
Specific functions proteoglycans
HA: Cell migration
CS: Formation of bone cartilage, cornea
KS: Transparency of cornea
DS:Transparency of cornea, Binds LDL
Heparin: Anticoagulant, cause release of Lipoprotein Lipase
HS: Component of skin fibroblass and aortic wall
Degradation of proteoglycans
ECM proteoglycans are ____
Endocytic vesicles fuse with ____
Lysosomal ___and____ degrade the core protein and carbohydrate components of the proteoglycans, respectively
_______ are a group of metabolic disorders caused by the ___ or ____ of different lysosomal enzymes needed to break down GAGs – results in ____ deformities
Degradation of proteoglycansECM proteoglycans are endocytosed Endocytic vesicles fuse with lysosomes Lysosomal proteases and glycosidases degrade the core protein and carbohydrate components of the proteoglycans, respectively Mucopolysaccharidosis are a group of metabolic disorders caused by the absence or malfunctioning of different lysosomal enzymes needed to break down GAGs – results in skeletal deformities
Interns
The linkage of the extracellular matrix to the cell requires transmembrane ___ ___ ___ that act as matrix ____ and tie the ___ to the cell’s ___
The principal receptors on animal cells for binding most extracellular matrix proteins—including ___ ____ ____—are the ___
Integrins play central roles in the biology by controlling___ ____ to the ECM and cell _____, ___, ____ and ____.
As a result, they contribute to the regulation of ____, ___, ____ and ___, and to the development of ___
Integrins differ from other cell surface receptors because they:
Bind their ligands with much____ affinities Are expressed ____fold more Why? If the binding were too tight, cells would become ___ ___ to the matrix and would be unable to ____—a problem that does not arise if attachment depends on large numbers of weak adhesions -“___ principle”
The linkage of the extracellular matrix to the cell requires transmembrane cell adhesion proteins that act as matrix receptors and tie the matrix to the cell’s cytoskeleton The principal receptors on animal cells for binding most extracellular matrix proteins—including collagens, fibronectin and laminins—are the integrins Integrins play central roles in the biology by controlling cell adhesion to the ECM and cell migration, growth, differentiation and apoptosis.
As a result, they contribute to the regulation of development, immunity, inflammation and hemostasis, and to the development of diseases Integrins differ from other cell surface receptors because they: Bind their ligands with much lower affinities Are expressed 10-100 fold more Why? If the binding were too tight, cells would become irreversibly glued to the matrix and would be unable to move—a problem that does not arise if attachment depends on large numbers of weak adhesions -“Velcro principle”
Structure of integrins
An integrin molecule is composed of ___ ______associated transmembrane ___ subunits called __ and __
The heterodimer integrin consists of a ____ extracellular (___) ligand-binding head, ___ ____-pass transmembrane helices and ___ ___cytoplasmic tails
A ____ of human integrin heterodimers are formed from 9 types of β subunits and 24 types of α subunits
Different integrins can bind the___ ____ ____. Ex: At least 5 different integrins bind laminin
xAn integrin molecule is composed of two noncovalently associated transmembrane glycoprotein subunits called α and β The heterodimer integrin consists of a large extracellular (ecto) ligand-binding head, two single-pass transmembrane helices and two short cytoplasmic tails A variety of human integrin heterodimers are formed from 9 types of β subunits and 24 types of α subunits Different integrins can bind the same ECM protein. Ex: At least 5 different integrins bind laminin
Activation of integrins and signaling
Integrins exist in an ____ between a __ ____-affinity state and an ___ ___-affinity state
When integrins are in the resting state, the α and β ____ tails are in ____ proximity, which constrains the integrin in a ____ _____conformation, in a bent form in which the head group points ____towards the cell surface
____ signals can activate the integrins and force them to “___ ___” (___-___ signaling)
During ‘inside–out’ signaling, an intracellular activator binds to the __ ___ ___, leading to conformational changes that result in ____affinity for extracellular ligands
In ‘outside–in’ signaling, binding of integrins to their___ ___ changes the conformation of the integrin and, because many of the ligands are multivalent, contributes to ___ ___.
Therefore, integrins are _____ signaling proteins. Inside-out and outside-in signaling occur in consortium
Activation of integrins and signaling Integrins exist in an equilibrium between a bent low-affinity state and an upright high-affinity state When integrins are in the resting state, the α and β cytoplasmic tails are in close proximity, which constrains the integrin in a low-affinity conformation, in a bent form in which the head group points inwards towards the cell surface Intracellular signals can activate the integrins and force them to “stand up” (inside-out signaling) During ‘inside–out’ signaling, an intracellular activator binds to the β-integrin tail, leading to conformational changes that result in increased affinity for extracellular ligands In ‘outside–in’ signaling, binding of integrins to their extracellular ligands changes the conformation of the integrin and, because many of the ligands are multivalent, contributes to integrin clustering Therefore, integrins are bidirectional signaling proteins. Inside-out and outside-in signaling occur in consortium
Functions controlled by integrin signaling
If bound to ECM components: _____ _____
If bound to ligand: ____ _____
If no ligand bound: ____
Functions controlled by integrin signaling
If bound to ECM components: Cell Adhesion Cell Migration
Cell Proliferation, survival and death
If no ligand boundà Apoptosis
Integrin-mediated cell adhesion and migration
Attachment of cells to ECM components induces ____ of integrins on the cell surface.
The cytoplasmic portions of the clustered integrins then act as a platform for the _____ of ____ proteins such as adaptor/scaffold and signaling proteins to the inner surface of the plasma membrane, where they form structures called ____ ____.
The adaptor/scaffold proteins in FAs provide ____ to the ___ ___ and, thereby, connect cells firmly to the ECM.
This linkage enables the generation of the ____ necessary to alter cell ____ and the____ force necessary to ___ the cell body during migration.
In addition, multiple signaling proteins, including __ or ____, are also recruited to FAs where they transmit ___-derived signals to cellular pathways controlling ___ ____ ____
Attachment of cells to ECM components induces clustering of integrins on the cell surface. The cytoplasmic portions of the clustered integrins then act as a platform for the recruitment of cellular proteins such as adaptor/scaffold and signaling proteins to the inner surface of the plasma membrane, where they form structures called focal adhesions. The adaptor/scaffold proteins in FAs provide linkages to the actin cytoskeleton and, thereby, connect cells firmly to the ECM. This linkage enables the generation of the tension necessary to alter cell morphology and the traction force necessary to move the cell body during migration. In addition, multiple signaling proteins, including kinases or phosphatases, are also recruited to FAs where they transmit ECM-derived signals to cellular pathways controlling proliferation, survival and migration.
Visualizing focal adhesion formation and actin dynamics in adhering and migrating cells
Visualizing focal adhesion formation and actin dynamics in adhering and migrating cells
Armed to the teeth with αvβ6 intern
Tooth enamel is the hardest mineralized tissue in the body and is produced by cells called ____.
During the secretory stage of amelogenesis, enamel proteins such as ____ are secreted into the enamel ____, which is in direct contact with the____ plasma membrane.
A recent study shows that integrin αvβ6 is expressed in _____ where it regulates ____ of ____ and subsequent enamel _____.
In mice deficient in αvβ6 integrin (Itgb6−/−), the researchers found ___ ____ ___,with a significantly reduced ___:____ratio, whereas the molars of these mice show reduced ____ and severe___.
Tooth enamel is the hardest mineralized tissue in the body and is produced by cells called ameloblasts. During the secretory stage of amelogenesis, enamel proteins such as amelogenin are secreted into the enamel matrix, which is in direct contact with the ameloblast plasma membrane. A recent study shows that integrin αvβ6 is expressed in ameloblasts where it regulates deposition of amelogenin and subsequent enamel biomineralization. In mice deficient in αvβ6 integrin (Itgb6−/−), the researchers found chalky rounded incisors, with a significantly reduced mineral-to-protein ratio, whereas the molars of these mice show reduced mineralization and severe attrition.
