Ch 17 Actin Filaments

What are major roles of actin filaments?

• maintaining cell shape

• enabling cell movements involving the plasma membrane

What type of protein is actin and how common is it?

its one of the most abundant proteins in almost all cell types

Actin filaments are

helical polymers

What actin-dependent process occurs in plant cells?

cytoplasmic streaming in the cell cortex

What motor protein is required for most actin-dependent movements?

myosin

How are actin monomers arranged in a filament?

as a twisted chain of identical globular monomers all pointing in the same direction

Do actin filaments have polarity?

yes; it has a plus end and a minus end

• like microtubules

How do actin filaments differ from microtubules in size?

they’re thinner, more flexible, and usually shorter

At which end of an actin filament does growth occur faster?

the plus end

What nucleotide does a free actin monomer carry?

ATP

How does ATP hydrolysis affect actin filament stability?

ADP-actin has weaker binding, decreasing filament stability and promoting depolymerization

What process in microtubules is analogous to ATP → ADP hydrolysis in actin?

GTP → GDP hydrolysis in microtubulin dimers

What happens to actin filament ends at intermediate monomer concentrations?

• plus end adds monomers

• minus end loses monomers

What happens to an actin filament when free monomer concentration is very high?

it grows rapidly at both ends

What is treadmilling?

simultaneous monomer addition at the plus end and loss at the minus end, causing monomers to “move through” the filament

Why does the plus end grow during treadmilling?

monomer addition outpaces ATP hydrolysis

Why does the minus end lose monomers during treadmilling?

ATP is hydrolyzed faster than new monomers can be added

• ADP-actin is unstable

What do most actin functions depend on?

dynamic assembly and disassembly controleld by monomer pool and actin-binding proteins

Why don’t actin monomers spontaneously polymerize in cells despite their high concentration?

actin-binding proteins prevent uncontrolled polymerization

What is the general role of actin-binding proteins?

they bind actin monomers and filaments to regulate when and where polymerization occurs

What do actin-related proteins (ARPs) do?

promote actin nucleation and polymerization

Why do cells sequester actin monomers?

to prevent premature polymerization and allow controlled filament assembly

Most actin-binding proteins bind to what form of actin?

assembled actin filaments

What type of protein family does myosin belong to?

a family of actin-binding motor proteins

What reaction provides energy for myosin movement?

ATP hydrolysis

In which direction does myosin move along actin filaments?

toward the plus end

What are the most abundant myosin subfamilies?

• myosin I

  • myosin II

Which myosin type is present in all cell types?

myosin I & II

What does the head of myosin I do?

binds actin and performs ATP-dependent motor activity

What determines the cargo that myosin I carries?

its tail domain

What are the functions of myosin I?

• moves vesicles along actin filaments

• binds and reshaped the plasma membrane

Which myosin type is present in all cells and has two heads?

myosin II

What does myosin II do inside cells?

• forms contractle bundles with actin

• drives cell shape changes

• movement

• division

Which myosin type is specialized in muscle cells?

a specialized form of myosin II

Where is actin highly concentrated in many eukaryotic cells?

in a layer beneath the plasma membrane called the cell cortex

What generates changes in cell shape within the cortex?

rearrangments of actin filaments and interactions wth myosin motors