Cytoskeleton and Intracellular traffic Flashcards
Function of intermediate filaments?
provide tensile strength to withstand mechanical stress
4 classes of intermediate filaments?
- keratin
- vimentin and vimentin related
- neurofilaments
- nuclear lamins
where are keratins located?
in epithelia
where are vimentin and vimentin related located?
connective tissue, muscle cells, glial cells
where are neurofilaments located?
nerve cells
where are nuclear lamins located?
all animal cell nuclei
how do the intermediate filaments in epithelium join cells together?
desmosomes and hemidesmisomes to allow for stretching and redistribution of applied forces
what is different from the intermediate filaments in the cytoplasm vs nucleus?
nuclear lamina have 2D mesh structure instead of ropelike structure
what are microtubules composed of?
alpha and beta tubulin heterodimers, form 13 protofilaments that are organized around a hollow core
what is the polarity of a microtubule?
plus end = beta tubulin
minus end = alpha tubulin
how do microtubules grow and shrink?
due to GTP hydrolysis
GTP packs tightly
GDP pack loosely (then dissociate) on + end
what do mitotic inhibitors do
?
target microtubules that inhibit their dynamics by either stabilizing or depolymerizing
what is an example of a stabilizing microtubule?
tau protein stabilizes microtubules
location of polarized ends of microtubules from centrosome?
+ end = periphery
- end = at centrosome
direction of kinesin travel?
toward plus end
direction of dynein travel?
minus end
examples of 2 microtubule associated molecular motors that are dependent on ATP?
kinesin, dynein
what cellular appendeges are formed by microtubules?
cilia and flagella
pathology of Kartagener syndrome?
defective celia in respiratory tract, fallopian tubes and sperm flagella due to mutation in gene encoding outer dynein arm
clinical triad of Kartagener syndrome?
situs inversus, chronic sinusitis, bronchiectasis
-reduced clearance of mucus from lungs (current RI), sterile males reduced fertility in females
What are G-actin?
actin subunits
what are F-actin
actin filaments
cellular functions of actin?
- make up microvili
- contractile bundles in the cytoplasm
- sheet-like and finger like protrusions on leading ends of motile cells
- contractile rings during cell division
main function of microvili?
enhances surface area of cell and increase rate of diffusion of materials into the cell
what makes up the base of microvili?
densely packed bundles of actin filaments?
what make up the base of cilia?
basal body
main difference between cilia and microvili?
cilia are motile OR non-motile, whereas microvili are NEVER motile (no wave like). cilia used to move cell or to move objects over surface of cell, cilia made of microtubules
what is the exception of motility for actin filaments?
not motile in muscle cells, because they are structurally stable
mechanism of growth of actin ?
grow from + end and simultaneously dissasemble from - end, called treadmilling
how do drugs/toxins modulate actin dynamics?
usually stabilize or prevent depolymerization or polymerization
what supplies energy to actin filaments?
ATP
what molecular motor is actin associated with?
myosin, most move toward + end of actin
function of tropomyosin?
2 filamentous molecules entwine with each other as a helix and lay end to end on actin filament and conceal myosin binding sites unless Ca binds to troponin and uncovers the myosin binding cites
What is spectrin?
a long filament protein that dimerizes and forms meshwork just underneath the plasma membrane
what is ankryn?
links spectrin to integral membrane protein band 3
what is band 4.1?
connects spectrin to actin and spectrin to glycophorin (integral membrane protein)
what does actin do in the cytoskeleton?
connects end of individual spectrin filaments
what combination of cytoskeletal filaments are the ONLY cytoskeletal components of erythrocytes?
spectrin, ankyrin, band 4.1, and actin
because they need to be able to deform to fit through small capillaries
pathology of hereditary spherocytosis?
mutation in genes encoding for components of erythrocyte cytoskeleton
mechanism of budding?
requires membrane fusion initated from lumenal side of membrane
mechanism of vesicle fusion?
requires membrane fusion event initated from the cytoplasmic side of both donor and target membrane?
three major types of coated vescicles?
clathrin, COPI, COPII
where do clathrin coated vesicles transport to and from?
from plasma membrane to golgi, from golgi to late endosomes, from secretory vesicles to golgi
where do COPI coated vesicles transport to and from?
retrograde transport from golgi to ER
where do COPII coated vesicles transport to and from?
transport from ER to cis golgi
what is SAR1?
the coat-recruitment GTPase responsible for COPII coat assembly at ER membrane
pathway of SAR1?
start in high concentrations in the cytoplasm in an inactive GDP bound state, then membrane bound Sar1-GEF protein binds to Sar1, causing it to release GDP and bind GTP instead. when GTP binds, Sar1 changes shape and inserts amphipathic helix into the membrane.
what does the amphipathic helix that is inserted into the membrane recruit?
2 components of the COPII coat: sec23 and sec24 which bind cytoplasmic tails of cargo receptors, sequestering the receptors and associated cargo
what other proteins are recruited after the cargo is sequestered?
sec13 and sec31 form outer shell of coat with Sar1 GTP and sec23/24 and membrane proteins as the inner shell
how do cargo receptors and membrane bound cargo proteins bind to COPII coats?
they have exit signal sequences in their cytosolic domains
what do SNAREs do?
they are surface bound molecules that fuse vesicles through homotypic membrane fusion that are transitioning from the ER to the cis golgi as VESICULAR TUBULAR CLUSTERS
how do vesicular tubular clusters move?
they move along microtubules (kinesins) toward cis golgi
what is the microtubule motor protein that assists with vesicles that move TOWARD golgi and plasma membrane?
kinesin
microtubular motor protein that moves vesicles toward golgi to ER from the plasma membrane?
dynein
how are ER cargo receptors and soluble ER proteins that are erroneously packaged into COPII vesicles returned to the ER via retrieval pathway?
COPI coats
what is the protocol for returning soluble ER resident proteins back to the ER?
these proteins contain specific amino acid sequence KDEL that is recognized by membrane bound receptors in the ER, and they are packaged into COPI coated vesicles and returned via KDEL receptors
what is the KDEL sequence?
lysine-aspartate-glutamate, leucine, recognizes soluble er resident proteins
what are clathrin coats made up of?
triskelions, 3 legged structure composed of 3 large and 3 small polypeptide chains forming a basket like structure on the cytosolic surface of membranes
what do clathrin coated vesicles do?
transport material from plasma membrane and between endosomal and golgi compartments, interact with adaptor protein on surface of membrane
what do adaptor proteins do?
form discrete second layer of coat positioned between clathrin cage and membrane, which bind to clathrin coat to the membrane, and trap transmembrane cargo receptors, and also package specific transmembrane proteins with soluble interacting proteins into once vesicles
what are Arf proteins?
coat recruitment GTPases that are responsible for COPI and clathrin coat assembly AT GOLGI MEMBRANES
what does COPII vesicle release requre?
it does not require dynamin, membrane fusion causes pinching off
what causes clathrin coated vesicle release?
dynamin that is localized at the neck of the bud (via PI(4,5)P2 binding domain), and uses GTP hydrolysis to bend the patch of membranes/distort bilayer to cause fusion of membranes on the neck and release the vesicle
what are Rab proteins?
proteins that display on the cytoplasmic surface of vesicles that specify target location of vesicle
how do Rab effectors work?
they bring vesicle close to target membrane to allow v-SNARE and t-SNARE interactions to fuse 2 membranes
what is a SNARE?
catalyzing membrane fusion reactions in vesicular transport and adds layer of specificity for Rab proteins/effectors
what are v-SNARES
found on vesicle membranes as a single polypeptide chain
what are t-SNAREs?
found on target membranes and are composed of 2-3 proteins
what occurs when the Rab protein and Rab effector get close?
helical domains of v-SNARE and t-SNARE wrap aroudn each other to form trans-SNARE complex that holds vesicle membrane close to target membraneq
what occurs in regulated exocytosis regarding SNAREs?
inhibitory proteins prevent v and t SNAREs from forming fully functional complex until a localized influx of Ca releases the inhibitory protein and allows the completion of the trans-SNARE complex and fusion of plasma membrane
go through endocytosis from the plasma membrane to the lysosome
- endocytic vesicles from plasma membrane bud off using clathrin coats and fuse with early endosomes
- then make multivesicular bodies and fuse with late endosomes
- late endosome fuse with lysosomes forming endolysosome
what is an early endosome?
found just inside plasma membrane and have pH of 6
what is a late endosome?
found closer to golgi and nucleus, have pH closer to 5
characteristic of lysosomes?
acidic inside filled with acid hydrolases that are active under acidic conditions that is maintained via V-type ATPases that pump H+ into the organelle
how are hydrolases transported to the lysosome?
- after M6P is added to the enzyme in the golgi, receptors for M6P are found on the membrane of trans golgi
- these receptors recruit adaptor proteins that bind to cytoplasmic tail and recruit clathrin coat
- budded vesicles transported to early endosome
- phosphate on enzyme is removed in the endosome so the enzyme is not brought back to the golgi
how does the lysosome degrade things?
- phagocytosis
- endocytosis
- autophagy
what is phagocytosis
engulfment of bacteria by phagocytic cells like macrophages
what is endocytosis
internalizing of extracellular molecules
what is autophagy
degradation of cellular components that are old, damaged or nonfunctional
example of receptor mediated endocytosis
- ferrotransferrin binds to transferrin receptor on surface and clathrin coated vesicle is formed
- clathrin coat dissociates
- vesicle fuses with early endosome
- early endosome fuses with late endosome
- ferric ions on ferrotransferrin are released d/t acidic environment
- apotransferrin remains bound to receptor
- vesicle buds off late endosome to form recycling endosome
- recycling endosome returns to cell surface with the apotransferrin
what could happen to endocytosed transmembrane receptor proteins?
- recycled via recycling endosomes
- transcytosis, where the receptor is delivered to a different membrane from which it came from , also mediated through recycling endosome
- degradation, aka degraded in the lysosome
2 functions of recycling endosomes?
- returning apoenzymes to plasma membranes
- storing transmembrane proteins/transported that can be quickly shuttled to plasma membrane in response to external stimuli
example of recycling endosome function in the cell
GLUT4 transporters ar stored inside recycling endosomes, so when insulin binds to insulin receptor, signaling cascade initiates the budding of GLUT4 transporter containing vesicles to the plasma membrane
where are secretory proteins packaged?
in the trans golgi via mechanism that involves selective aggregation of the secretory proteins to be packaged into mature secretory vesicles
example of secretory proteins packaged
beta cells in pancreas have secretory vesicles that are packed with insulin
mechanism of insulin secretion in the pancreas
- glucose influx to beta cells via facilitated diffusion via GLUT transporter
- glucose metabolized in the cell, mitochondria produces ATP
- ATP closes ATP-sensitive K channel on plasma membrane
- inhibition of K channel causes depolarization of membrane (more +++ charge)
- depolarization opens voltage gated Ca channels, Ca influx
- Ca induces release of insulin secretory granules
MOA of sulfonylurea class of drugs?
inhibits ATP sensitive K channel, causing depolarization and increase in insulin release from beta cells
example of sulfonylurea?
glimepiride, glipizide, glyburide
what organelle organizes microtubules?
the centrosome, specifically the centrioles and pericentriole (gamma tubulin rings where microtubules originate from)
