Test 3. Lecture 27

Question 1

Actin filaments are organized into:
•__________—filaments are cross-linked
into closely packed parallel arrays.
• ____________—filaments are cross-linked
in arrays that form 3-D meshworks with the
properties of semisolid gels.

Answer 1

Actin bundles

Actin networks

Question 2

cross-linking protiens have at least two domains that bind actin

__________are small, rigid proteins that force filaments to align
closely.
Proteins that organize actin filaments into networks tend to be large flexible proteins
that cross-link perpendicular filaments.

Answer 2

Actin-bundling proteins

Question 3

Two types of actin bundles:
1. __________—closely spaced filaments aligned in parallel, with the same
polarity, with barbed ends adjacent to the plasma membrane.
__________ is a bundling protein in parallel bundles; first isolated from INTESTINAL MICROVILLI

2.___________—more widely-spaced filaments, cross-linked by
_________
Increased spacing between filaments allows the motor protein myosin to interact
with the actin filaments.

Answer 3

Parallel bundles
Fimbrin

Contractile bundles
α-actinin.

Question 4

In actin networks, proteins
such as \_\_\_\_\_\_\_\_\_\_ form
flexible cross-links.
A filamin dimer is a flexible
\_\_\_\_-shaped molecule with
actin-binding domains at
the end of each arm.

Answer 4

filamin

V- shaped

Question 5

Actin filaments are concentrated AT CELL PERIPHERY where they form a 3-D
network beneath the plasma membrane.

This network and associated proteins (________) determines cell
shape and is involved in activities such as movement.

Red blood cells (erythrocytes) are useful for studies of the cortical
cytoskeleton.

They have no nucleus or organelles, so plasma membranes and associated
proteins are easily isolated.

They also lack other cytoskeletal components, so the cortical cytoskeleton is
the principal determinant of cell shape

Answer 5

the cell cortex

Question 6

__________ is a member of the calponin family of actin-binding proteins.
It is a tetramer of two polypeptide chains, α and β. The ends of the spectrin
tetramers associate with short actin filaments, resulting in the spectrinactin
network.

Answer 6

Spectrin

Question 7

__________links the spectrin-actin network and the plasma membrane by
binding to spectrin and a transmembrane protein (band 3).

Answer 7

Ankyrin

Question 8

Most cells have specialized plasma membrane regions that form contacts with :
1.
2.
3.

These regions are also attachment sites
for actin bundles, evident in fibroblasts
maintained in tissue culture.
Cultured fibroblasts secrete extracellular
matrix proteins that stick to the dish.

Answer 8

• adjacent cells,
• the extracellular matrix, or
• other substrata (such as the surface of a culture dish).

Question 9

The fibroblasts attach to the matrix via binding of transmembrane proteins
(________)
.
The sites of attachment (___________) are also attachment sites for large actin bundles called ____________

___________, cross-linked by α-actinin and stabilized by tropomyosin.

Two other proteins,______ and ________ are involved in binding stress fibers.

Answer 9

integrins

focal adhesions

stress fibers.

Stress fibers are contractile bundles

talin and vinculin

Question 10

“attachment of actin filaments to adherens junctions”

In sheets of epithelial cells, cell–cell contacts (adherens junctions) form a
continuous ___________around each cell.

Answer 10

adhesion belt

Question 11

“attachment of actin filaments to adherens junctions”

Contact is mediated by transmembrane proteins called ________. They
bind to cytoplasmic _________-, which anchor actin filaments to the plasma
membrane.

Answer 11

cadherins

catenins

Question 12

 Cell surfaces have a variety of protrusions involved in cell movement, phagocytosis, or functions such as absorption of
nutrients.

 These extensions are based on _________, in relatively permanent or
rapidly rearranging bundles or networks.
 ___________ are fingerlike extensions;
abundant on cells involved in absorption.
1. Microvilli of epithelial cells lining the
intestine.
2. They increases the surface area for
absorption by ten to twentyfold.

Answer 12

actin filaments

Microvilli

Question 13

Intestinal microvilli contain closely
packed parallel bundles of 20 to 30
________________.

Answer 13

actin filaments

Question 14

The filaments are cross-linked by
__________ and __________

The actin bundles are attached to the
plasma membrane by the Cabinding
protein calmodulin in
association with myosin I.

Answer 14

fimbrin and villin.

Question 15

Other surface protrusions are transient and form in response to
environmental stimuli.

____________ are extensions of moderate width, responsible for
phagocytosis and the movement of amoebas.

_____________ are broad, sheetlike extensions at the leading edge of
fibroblasts.

Many cells also extend _____________, thin projections of the plasma membrane
supported by actin bundles.

Answer 15

Pseudopodia

Lamellipodia

filopodia

Question 16

Cell movement or extension of cellular
processes involves a coordinated
series of movements.

Cell locomotion
Movement across a surface proceeds in 3
stages: 
1.
2.
3.

Answer 16

– Extension of the leading edge
– Attachment of leading edge to the
substratum
– Retraction of the rear of the cell
into the cell body

Question 17

 Formation of cell surface protrusions in response to extracellular stimuli
is regulated by  Formation of cell surface protrusions in response to extracellular stimuli
is regulated by ____________

Answer 17

Rho proteins

Question 18

Rho proteins activate ___________, which stimulate the __________
complex and initiate growth of branched actin filaments.

Answer 18

WASP proteins

Arp2/3

Question 19

1. _________ is a __________—a protein that converts chemical energy (ATP) to
   MECHANICAL ENERGY, generating force and movement.
2. Skeletal muscles are bundles of ___________, large cells formed by fusion of
   many cells during development.
3. Most of the cytoplasm consists of _____________, BUNDLES OF THICK MYOSIN FIlAMENTS AND THIN ACTIN FILAMENTS
   and thin actin filaments.

Each myofibril is a chain of contractile units called __________, which give skeletal
and cardiac muscle their striated appearance.

Answer 19

1. Myosin
   molecular motor
2. muscle fibers
3. myofibrils

sarcomeres

Question 20

Electron microscopy

1. Actin Filaments are attached at their ________ to the _________
2. myosin filaments are anchored at the __________
3. I band, only actin filaments; A band a myosin filaments; overlap in the peripheral region of the A band

Answer 20

1.barbed ends
z disk
\
2. M line

Question 21

The __________________ of muscle contraction
1. during contraction, _______ shorten, bringing _______ closer

2. no change in width of ___________, but _________ almost disappear
3. The actin and myosin filaments slide past one another so that the actin filaments move into A band and H zone
4. The molecular basis for this is the binding of myosin to actin filaments, allowing myosin to function as a motor that drives filament sliding

Answer 21

1. During contraction, sarcomere shortens, brining z disc closer together.
2. no change in width of A band, but I bands and H zone allmost disappear

Question 22

MYOSIN II: ______________, has 2 heavy chains and 2 pairs of light chains

• the heavy chains have a globular head region and a long alpha helical tail
• the tails twist around each other in a coiled coil

Answer 22

the type in muscle

Question 23

” organization of actin and myosin filament”

_____________: several hundred myosin molecules in parrallel staggered array

_____________ , forming cross-bridges between thick and thin filaments

the oriientation of filaments reverses at the M line

Answer 23

thick filaments

globular heads bind actin