BMB 252 - Cell and Molecular Biology II - Exam II

Fall 2026 - Penn State - Dr. Maria Krasilnikova

Intermediate filaments, microtubules, and actin filaments

The three main types of filaments in the cytoskeleton

Provide mechanical strength

Function of intermediate filaments in the cytoskeleton

Determine the position of membrane-enclosed organelles, and direct intracellular transport

Function of microtubules in the cytoskeleton

Determine the shape of the cell surface, necessary for cell movement

Function of actin filaments in the cytoskeleton

Mitotic Spindle

What major machine for cell division is composed of microtubules?

Contractile Ring

What major machine for cell division is composed of actin filaments?

Form microvilli for absorption of nutrients

Function of actin filaments in epithelial cells

Provide connection to the basal lamina and between cells

Function of intermediate filaments in epithelial cells

Direct newly synthesized components

Function of microtubules in epithelial cells

Tubulin Subunits

What subunit makes up microtubules?

Actin subunits

What subunit makes up actin filaments?

Intermediate filament proteins

What subunit makes up intermediate filaments?

Subunits

What structures assemble into protofilaments?

Protofilaments

What structures assemble into filaments?

Microtubules and actin filaments

What filament(s) is/are polar?

Intermediate filaments

What filament(s) is/are non-polar?

Plus End

Which end of a filament is more dynamic?

Minus End

Which end of a filament has more stably connected subunits?

Profilin

What protein can aid in the attachment of actin subunits to form actin filaments?

Thymosin

What protein binds to actin subunits and inhibits their ability to form actin filaments?

Stathmin

What protein blocks the binding of tubulin subunits, inhibiting their ability to form microtubules?

ATP

What substance does each actin subunit have a binding site for?

Two parallel protofilaments twist around each other in a right-handed helix

How do actin filaments form?

Actin subunits

Which filament subunit has ATP?

Tubulin subunits

Which filament subunit has GTP?

They are hydrolyzed to ADP/GDP

What happens to ATP/GTP upon addition to a filament?

Treadmilling

The phenomenon where, at some concentration of free subunits, the plus end grows and the minus end shrinks at the same rate, so the length of the filament stays the same

At or near the plasma membrane

Where does actin filament nucleation typically occur?

The cell periphery

Where is the highest density of actin?

Actin filaments in the cell cortex

What structure(s) determine the shape and movement of the cell surface?

ARP complex and formins

What two types of regulated factors promote actin nucleation?

Branched actin structures

What type of structures do ARPs catalyze the formation of?

Non-branched actin filaments

What type of structures do formins catalyze the formation of?

Minus end

What end of the actin filament do ARPs nucleate actin at?

Forming a ring over the plus end

How do formins stabilize actin filaments?

Formins

Which actin-filament-nucleating protein helps to organize the contractile ring in mitosis?

Capping Proteins

What can be added to the end of an actin filament to stabilize it, not allowing it to shrink or grow?

ARPs and y-TuRC

What are the two major examples of capping proteins for minus ends on actin filaments?

Bind to polarized cytoskeletal filaments, and use energy from ATP hydrolysis to move along the filament

What is the general method of action of motor proteins?

Motor domain, binds to a filament and hydrolyzes ATP

What is the function of the head region of a motor protein?

Binds to the cargo

What is the function of the tail region of a motor protein?

Carrying cargo, and causing cytoskeletal filaments to slide against each other

What are the two major functions of motor proteins?

Myosins

Major motor protein(s) associated with actin filaments

Plus end

What direction of the actin filament do myosins move toward?

Kinesins and Dyneins

Major motor protein(s) associated with microtubules

Mostly the plus end

What direction of the microtubule do kinesins move toward?

Minus End

What direction of the microtubule do dyneins move toward?

Myosin V

Which myosin is associated with organelle and vesicle transport?

Myosin II

Which myosin is responsible for contractile activity in muscle and non-muscle cells?

2 Heavy Chains and 2 Light Chains

What is the structure of myosin II?

Globular head domains (ATPases) and extended coil-coil domain for dimerization

What do the heavy chains that help make up myosin II have that make them special?

Each myosin head binds and hydrolyzes ATP, and the energy from the hydrolysis is used to move the myosin heads toward the + end of the actin filament, resulting in the sliding of actin filaments relative to myosin

How does myosin ii enact the skeletal muscle contraction?

Rigor configuration

During what stage of myosin movement / muscle contraction is the myosin head attached to actin?

ATP binding to myosin

What causes the myosin head to release from actin, coming out of rigor configuration?

ATP hydrolysis

What process is responsible for the myosin head swinging into the cocked conformation?

Phosphate released (force-generating step / power stroke)

What happens once the myosin head is bound to actin again?

ADP is released and the myosin head returns to its original attached conformation

What happens after the power stroke in myosin movement?

A dark band and half of a light band on each side

What is every sarcomere made up of?

Dark bands

Central element of the sarcomere, composed of thick filaments of myosin II

Light bands

Element of a sarcomere, composed of actin

Z-Disk

Place of attachment of actin filament plus ends

Titin

Protein in the sarcomere that acts as a molecular spring, allowing the muscle fiber to return to its initial state

Calcium (Ca2+)

What ion is found in low concentrations in resting muscle, but high concentrations in contracting muscle?

Troponin pulls tropomyosin away from its normal binding groove into a position that interferes with the binding of myosin heads

What is the state of troponin and tropomyosin in resting muscle?

Calcium binds to troponin, causing it to release tropomyosin. Tropomyosin slides into a position that does not interfere with myosin binding

What is the state of troponin and tropomyosin in contracting muscle?

Nerve signals reach T tubules, which release calcium, binding to receptors on the sarcoplasmic reticulum, leading to a massive calcium release

What signal causes calcium release from the sarcoplasmic reticulum?

Acetylcholine

The influx of calcium triggers the release of what neurotransmitter into the synaptic cleft during muscle contraction?

Organelles, golgi stacks, secretory vesicles, and RNA

What kind of cargo can motor proteins move?

Motor domain

What domain is the same in all myosins?

Rigor Conformation

What conformation of the myosin filament is responsible for rigor mortis?

Muscle Fiber

Is a muscle cell a muscle fiber or myofibril?

Nicotinic Receptor

What kind of receptor on a muscle cell does acetylcholine interact with?

13 parallel protofilaments

What is the microtubule composed of?

It is trapped and cannot be exchanged or hydrolyzed

What is true of the GTP inside of the alpha-tubulin?

GTPase

What kind of enzyme does beta-tubulin function as?

GTP

What molecule does every tubulin subunit have a site for the binding of?

Alpha-tubulin

Which microtubule subunit faces the minus (-) end?

Beta-tubulin

Which microtubule subunit faces the plus (+) end?

Protofilaments containing GTP are straight while those with GDP are curved

What is the difference between protofilaments with GTP and those with GDP?

It is more likely to be stable and add more subunits

What is true of the GTP-bound end of a microtubule?

It is more likely to be unstable and lose subunits

What is true of the GDP-bound end of a microtubule?

MAPs (microtubule-associated proteins)

What class of proteins bind along the sides of microtubules?

MAP2

The free end of what protein can contact MAP-coated microtubules and organize them?

Rescue

What is the term for the GTP-bound end of a protofilament adding subunits?

Catastrophe

What is the term for the GDP-bound end of a protofilament shrinking?

Dynamic Instability

What is the phenomenon of the plus end of protofilaments changing from growing to shrinking and back called?

y-tubulin

What substance regulates the nucleation of microtubules?

Microtubule-Organizing Center (MTOC)

At what body are microtubules nucleated?

Its minus end

Where on the microtubule is it nucleated?

y-tubulin ring complex (y-TuRC)

What complex serves as a template to assemble new tubulin subunits into the filament?

Centrosome

What is the MTOC (microtubule organizing center) known as in most animal cells?

A fibrous centrosome matrix, more than 50 copies of y-TuRC, and two centrioles at a right angle relative to each other

What structures are contained within the centrosome?

A short cylinder of modified microtubules and many accessory proteins

What is the composition of a centriole?

Myosin II

What other motor protein do kinesins have a similar structure to?

Kinesin-14

What kinesin variant moves to the minus end?

Kinesin-13

What kinesin variant is a catastrophe factor that breaks down motor tracts instead of traveling along them?

Kinesin-5

What kinesin variant is for sliding microtubules in an opposite direction

Kinesin-3

What kinesin variant carries membrane-enclosed organelles?

C domain

What domain of kinesin-1 is for carrying cargo?

Motor Domain

What domain is similar in all kinesins?