SFOM - Cytoskeleton, Endocytosis and Exocytosis {2.01,2.03}

What is the general structure of microtubules?

Hollow tube with a wall consisting of typically 13 protofilaments 

What are the monomers of microtubules?

Alpha-tubulin and beta-tubulin

What is the nucleotide substrate of microtubules?

GTP

What is the function of cytosolic microtubules?

organization/maintenance of shape and polarity, chromosome movement, intracellular transport

What is the function of axonemal microtubules?

Cell motility

What is the general structure of microfilaments?

two intertwined chains of F-actin

What is the monomer of microfilaments?

G-actin

What is the nucleotide substrate of microfilaments?

ATP

What are the functions of microfilaments?

Muscle contraction, cell locomotion, cytoplasmic streaming, cytokinesis,

What is the general structure of intermediate filaments?

Eight protofilaments joined end to end with staggering overlaps

What are the monomers of intermediate filaments?

six classes of proteins

What are the functions of intermediate filaments?

Structural support, maintenance of shape, formation of nuclear lamina, strengthening of nerve cell axons, keeping myofivrils in register

What does the term treadmilling mean?

occurs when the rate of addition of new subunits to the + end of a microtubule equals the rate of depolymerization at the - end

What are the steps of microtubule formation?

Alpha and beta-tubulin form heterodimers→heterodimers form oligomers→oligomers assemble into microtubules→treadmilling

What will happen to the size of a microtubule when concentration of free alpha/beta subunits >> critical concentration?

The microtubule will grow in size

What will happen to the size of a microtubule when concentration of free alpha/beta subunits << critical concentration?

The microtubule will shrink in size

What happens when there is no GTP cap at the + end of microtubules?

Rapid depolymerization of the microtubule

What is the general function of microtubule-organization centers(MTOCs)?

initiate the assembly of microtubules and act as an anchor for them

Which side of the microtubule remains anchored to the MTOC?

Negative end

What is the MTOC in ciliated cells?

basal body

What is the function of microvilli microfilaments?

Involved in absorbing nutrients in the intestine, increade surface area

What is the function of the cell cortex microfilaments?

Meshwork of fibers that supports the inner face of the plasma membrane, also serves as a support and staging area for membrane proteins

What is the function of the adherens belt microfilaments?

Helps to provide strength to the epithelia

What is the function of filopodia and lamellipodium microfilaments?

Involved in cell migration and movement

What is the function of stress fiber microfilaments?

Cellular contractility which plays a role in cell attachment, actin found in nonmuscle cells

What is the function of phagocytotic microfilaments?

Use F-actin to engulf cells

What is the function of endocytotic vesicle movement microfilaments?

F-actin involved in moving endocytotic vesicles away from the plasma membrane

What is the function of contractile ring microfilaments?

involved in physical splitting of one cell into two cells

What microfilament is number 1?

Microvilli

What microfilament is number 2?

Cell cortex

What microfilament is number 3?

Adherens belt

What microfilament is number 4?

Filopodia

What microfilament is number 5?

Lamellipodium/leading edge

What microfilament is number 6?

Stress fibers

What microfilament is number 7?

Phagocytosis

What microfilament is number 8?

Moving endocytic vesicles

What microfilament is number 9?

Contractile ring

What is the role of myosin in microfilament action?

motor protein, muscle contraction, contractile splitting of cells

What is the role of spectrin in microfilament action?

Provide strength and structure to certain cell types like erythrocytes

What is the role of dystrophin in microfilament action?

Structural protein that links actin polymers to glycoproteins so the cell sticks to basal lamina

What is the etiology of muscular dystrophy?

defect in DMD gene that codes for dystrophin so skeletal muscles do not bind to basal lamina

How is viscosity of the cytosol increased through actin?

polymerization is inhibited and F-actin is broken into smaller pieces decreasing viscosity

Why would we want a cell to be less viscous?

Makes the cell more easily engulf bacteria

What is the etiology of hereditary spherocytosis?

anemia caused by malformation of RBCs by then losing their microfilament structure

Where are hard keratins found?

hair and nails

Where are soft keratins found?

epithelial cells, mainly the skin

What is the etiology of epidermolysis bullosa simplex?

Point mutations in keratin proteins that cause skin fragility

What are keratins and lamins considered?

Intermediate filaments

What are lamins?

meshwork of intermediate filaments that line the nuclear envelope providing strength and support to the inner surface of the nuclear membrane

How do lamins contribute to the structural integrity of the nucleus?

Make contact with nuclear membrane proteins and attach in a mesh like pattern

What is the etiology of Hutchinson-Gilford Progeria syndrome?

Mutation in the gene encoding Lamin A, nuclear membrane integrity compromised which causes accelerated aging

What is phagocytosis?

Cell uses its membrane to engulf large pieces of foreign material

What is pinocytosis?

Non-specific mode of transport for extracellular fluid and solutes into cells

What is receptor mediated endocytosis?

Used to transport large molecules into human cells using receptors present in the plasma membrane

What are the steps of phagocytosis?

Phagocyte membrane surrounds microbe→microbe ingested by phagosome→fusion with lysosome→killing of microbes

What is Clathrin?

Coat protein that is required to mediate the formation and invagination of pits that contain material to be ingested

What are the steps of receptor mediated endocytosis?

Ligand binds to its receptor→receptor ligand complexes diffuse laterally→membrane invaginates and becomes coated→coated vesicles pinches off→clathrin coating is shed→naked vesicle fuses with early endosome→splits into two or more pieces with different end points

What is the function of dynamin in receptor mediated endocytosis?

Releases/cleave coated vesicle from the plasma membrane

What most likely aids the transport of vesicles in receptor mediated endocytosis?

Dyneins

What is an early endosome?

Pre-formed, largely empty organelle waiting to receive and sort materials

What is transcytosis in receptor mediated endocytosis?

Some or all of the original ligand is expunged from the cell from the early endosome

How are receptors reused in receptor mediated endocytosis?

Receptors are pulled from the early endosome and brought back to plasma membrane

How does an early endosome become a late endosome?

fusion with lysosomes for degradation

What is the function of kinesin in transport of vesicles?

Pull vesicles along microtubules, mainly to the periphery

What is anterograde transport?

From cis golgi to trans golgi

What is retrograde transport?

From trans golgi to cis golgi

What is the function of coat protein-1 (COPI)?

Involved in retrograde transport

What is the function of coat protein-2(COPII)?

Involved in anterograde transport

What is the function of ARF1?

Stimulated by GEF to assemble the clathrin coat and attach to cargo in the golgi

What happens to ARF1 after the vesicle leaves the golgi?

ARF1 molecules will hydrolyze their bound GTP to GDP, causing the Clathrin coat to fall apart.

T/F: vesicles can fuse to the plasma membrane when coated with clathrin.

False

What are v-SNARES?

docking receptors embedded in the membranes of vesicles

What are t-SNARES?

Docking receptors in target membranes

What is the function of effector proteins in vesicle fusion with target membrane?

Tethering proteins between SNARES

What is the function of Rab GTPases in vesicle fusion with target membrane?

stablish specificity in vesicle fusion and lock down vesicles onto target membranes

What is the function of the NSF/SNAP complex in vesicle fusion with target membrane?

Disassemble SNARE protein complexes once fusion is complete

What do kinesins and dyneins move along?

microtubule “railroad”

What does the head domain of kinesins interact with?

Microtubules, “walking action”

What does the light chain/tail region of kinesins interact with?

hold onto vesicle/cargo

In which direction do kinesins move?

towards positive end of microtubules

What is the function of cytoplasmic dyneins?

responsible for movement of vesicles towards the ends of chromosomes in human cells (retrograde transport)

Which end of microtubules do cytoplasmic dyneins move?

Negative end

What is the function of motor proteins in axons and dendrites?

Moving neurotransmitters along the length of axons

What is the function of motor proteins in positioning of organelles?

Move them to the right spots, ie. continuous pull on ER to prevent dissolution of golgi

What is the function of kinetochore microtubules in mitotic chromosome movement?

interact with the chromosomes at their kinetochore

What is the function of astral microtubules in mitotic chromosome movement?

Help anchor centrosomes in place

What is the function of polar microtubules in mitotic chromosome movement?

Stretch out towards the center of the dividing cell and overlap

Where does chromosomal microtubules attach in mitotic chromosome movement?

ends of chromosomes

What is the function of chromokinesin in mitotic chromosome movement?

drags chromosomes to the metaphase plate in the middle of the cell