Actin I
Actin II
Actin III
Actin IV, Intermediate Filaments
Tissues and the Extracellular Matrix
100
Fill in the blanks. The actin subunit is _____(a)_____ (highly conserved amino acid sequence - 88% identical between yeast and animal actin) _____(a)_____ polymerizes into thin filaments of _____(b)_____, which forms a right-hand helix (13 actin monomers per turn; 37 nm per turn). Actin has plus ends and minus ends
(a) G-actin (b) F-actin
100
Name two actin nucleators and give their functions.
Arp 2/3: nucleates branched networks Formin: nucleates bundled, unbranched cables
100
actin-rich cables can generate protruding spikes called what?
Filapodia (arrowheads)
100
Fill in the blanks about the transport of pigment granules. Cells that produce pigment for our skin and hair are called _____(a)_____. Pigment granules are also called _____(b)_____, specialized organelles closely related to lysosomes. Pigment producing enzymes are localized to _____(b)_____ and so these organelles fill up with the pigment melanin. The _____(b)_____ will travel down microtubules using a _____(c)_____ motor. Near the end of the process, the _____(b)_____ will hop over to an actin filament and use the myosin Va motor to move into the very tip of the process. The tips will bud off from the cell carrying a bunch of pigment granules - these packets of pigment are then endocytosed by other cells (skin cells or follicle cells that get incorporated into your hair).
(a) Melanocytes (b) Melanosomes (c) Kinesin
100
What makes up the basement membrane? What is the basement membrane's more accurate name?
The basement membrane is made of dense extracellular matrix material; Basal lamina
200
The growing end of actin will have what bound, similar to how the growing end of microtubules have GTP bound?
ATP
200
Why do actin filaments need nucleators?
Spontaneous assembly of actin filaments from G-actin subunits is very slow and so nucleators are needed.
200
The actin contractile bundles will help pull the pack edge of the cell forward using what?
Myosin motors; Actin can generate movements by motors and assembly.
200
Give one example of an intermediate filament.
Keratin
200
The extracellular matrix is the primary component of the basement membrane and what else?
loose connective tissue, cartilage, ligaments, tendons and bone.
300
What is another name for the barbed end of an ectin filament? The pointed end?
Plus-end; minus-end
300
ARP2/3 “nucleates” actin filament assembly by providing a template or “seed” that can be elongated by subunit addition. Arp2/3 has function that is similar to what in microtubules? What activates Arp2/3?
y-tubulin; WASP
300
What is another name for Type II myosin? Why is Type II myosin bipolar?
Conventional myosin; Bipolar because the heads on both sides of the myosin filament will pull actin towards the center.
300
Intermediate filaments provide structural support. What is their polarity?
They have no polarity
300
The extracellular matrix (ECM) is made up of what four things?
collagen, proteoglycans, fibronectin, laminin
400
Fully describe actin treadmilling.
An equilibrium can be established where the polymer length remains the same because the rate of loss from the minus end equals the rate of addition onto the plus end. This equilibrium condition is called “treadmilling” because the monomers are added to the plus end, move down the filament, and are released from the minus end.
400
Actin polymerization can drive what?
Cell motility; Actin assembly at the leading edge of a crawling cell can move the cells forward
400
Fill in the blanks. Each muscle fiber is made up of a bundle of myofibrils. Myofibrils consist of a repeating linear array of contractile units called _____(a)_____. A muscle fiber (cell) from a bicep may contain 100,000 _____(a)_____. A _____(a)_____ extends from Z-line to Z-line, and is made up of actin, myosin and a few other associated proteins.
(a) Sarcomere
400
True or false? Intermediate filaments require ATP, but not GTP, for assembly.
False; intermediate filaments require neither ATP nor GTP.
400
What is the most abundant protein in our bodies?
Collagen
500
Fill in the blanks about the regulation of actin assembly and disassembly during directed cell motility. Chemoattractant binds to cell surface receptor, which activates _____(a)_____. _____(a)_____ activates ARP2/3. Arp2/3 nucleates _____(b)_____, which grow from the plus (barbed) end. Arp2/3 nucleates branches. Filaments not touching the membrane are capped, those in contact with membrane push it outward. _____(c)_____ ends on the back edge are disassembled with the help of cofilin. Actin monomers can now diffuse to the front of edge and assemble again. When the activating signal (chemoattractant) is lost, new polymerization stops, the actin is disassembled and the cell stops moving in this direction.
(a) WASP (b) actin filaments (c) Minus
500
Formin forms unbranched cables. Unlike the Arp 2/3 complex, which stays with the minus end, where does formin stay?
The plus end
500
Describe Myosin V.
Myosin V is a walking motor with really long strides
500
Which intermediate filament is specifically required in the dividing cells at the base of the epidermis that attach to the underlying dermis?
Keratin 14; Without keratin, skin becomes unstable and even mild rubbing or abrasions could cause blistering (EBS).
500
What unique posttranslational modification does collagen contain that is needed for it to fold into a long, tough, triple helical structure?
hydroxyproline and hydroxylysine
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