Term
| What are the smallest of the cytoskeletal filaments? |
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Definition
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Term
| What are microfilaments best known for? |
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Definition
| their role in muscle contraction |
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Term
| What are some roles of microfilaments? |
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Definition
-muscle contraction
-cell migration and movement; cytokinesis
-maintenance of cell shape (used heavily in cell cortex)
-they form the structural core of microvilli (fingerlike extensions on epithelial cells) |
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Term
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Definition
a very abundant protein in all eukaryotic
cells (375 amino acids, 42 kDa) |
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Term
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Definition
G-actin-ATP molecules polymerize to form
microfilaments, F-actin |
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Term
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Definition
Once actin is synthesized, it folds into a globular-shaped
molecule that can bind ATP or ADP
(G-actin; globular actin) |
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Term
| What are the two divisive categories of actin? |
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Definition
Actins can be divided into muscle-specific actins
(alpha-actins) and nonmuscle actins (beta- and
gamma-actins) |
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Term
| Which types of actin localize to different parts of the cell? |
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Definition
| beta- and gamma-actin localize to different regions of a cell |
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Term
| Are F-actin filaments composed of actin monomers or dimers? |
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Definition
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Term
| What is the structure of an F-actin filament? |
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Definition
F-actin filaments are composed of two linear
strands of polymerized G-actin, wound into a helix; All the actin monomers in the filament have the
same orientation |
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Term
| Describe the polymerization of G-actin into filaments |
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Definition
G-actin monomers can polymerize reversibly into
filaments with a lag (nucleation) phase, and
elongation phase, similar to tubulin assembly |
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Term
| How can myosin be used to show the polarity of actin filaments? |
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Definition
• Myosin subfragment 1 (S1) can be incubated
with microfilaments (MFs)
• S1 fragments bind and ‘decorate’ the actin MFs
in a distinctive arrowhead pattern
• The plus end of an MF is called the barbed end
and the minus end is called the pointed end,
because of this pattern |
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Term
| How does the polarity of MFs affect their growth? How does this relate to the presence of either ATP or ADP bound to the G-actin monomers? |
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Definition
• Myosin subfragment 1 (S1) can be incubated
with microfilaments (MFs)
• S1 fragments bind and ‘decorate’ the actin MFs
in a distinctive arrowhead pattern
• The plus end of an MF is called the barbed end
and the minus end is called the pointed end,
because of this pattern |
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Term
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Definition
fungal metabolites that
prevent the addition of new monomers to the
plus end of MFs, leading to depolymerization |
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Term
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Definition
a sponge toxin that sequesters
actin monomers and prevents their addition to
MFs, also resulting in depolymerization |
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Term
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Definition
stabilizes (caps) MFs and prevents
their depolymerization |
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Term
| If cells crawl, what do they use to move? |
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Definition
Cells that crawl have lamellipodia and filopodia at
their leading edge, allowing them to move along a
surface |
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Term
| If cells adhere tightly to the underlying substratum and don't move well, what kind of microfilaments do they contain? |
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Definition
-Cells that adhere tightly to the underlying
substratum and don’t move well have organized
actin bundles running from rear to front called
stress fibers
• Activation of the RhoA pathway results in formation
of stress fibers (by activating formin, which helps
form long, unbranched MFs) |
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Term
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Definition
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Term
| How are stress fibers organized? |
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Definition
-Contractile bundle; MFs assembled pointing in opposite directions (positive/negative/positive, etc.)
• Activation of the RhoA pathway results in formation
of stress fibers (by activating formin, which helps
form long, unbranched MFs) |
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Term
| How is cell cortex assembled? |
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Definition
-Gel -- MFs arranged chaotically
-In rapidly moving cells, the cell cortex, just
beneath the plasma membrane, has actin
crosslinked into a gel or loose lattice |
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Term
| How are filopodia arranged? |
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Definition
-MFs are parallel to each other; all MFs are pointing in the same direction.
-In filopodia, at the leading edge,
microfilaments form highly oriented, polarized
bundles with the plus ends oriented toward
the tip of the protrusion |
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Term
| How are lamellipodia organized? |
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Definition
The actin in lamellipodia is less well organized
than in filopodia |
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Term
| How are rapidly organized cells organized? |
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Definition
| Rapidly moving cells form lamellipodia and filopodia; filopodia are at the leading edge while lamellipodia are close to the leading edge. |
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Term
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Definition
• Cells can precisely control where actin assembles
and the structure of the resulting network
-Actin-Binding Proteins Regulate the Polymerization, Length, and Organization of Actin |
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Term
| NEED TO MAKE FLASHCARDS FOR MICROTUBULES |
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Definition
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Term
| How do cells control where actin assembles and the structure of the resulting network? |
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Definition
• Control occurs at the nucleation, elongation, and
severing of MFs, and the association of MFs into
networks or bundles |
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Term
| What will happen to cells if the concentration of ATP-bound G-actin is high? |
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Definition
If the concentration of ATP-bound G-actin is
high, microfilaments will self-assemble until
the G-actin is limiting |
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Term
| Why is a large amount of free G-actin not available in cells? |
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Definition
In the cell, a large amount of free G-actin is not
available because it is bound by thymosin b4 |
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Term
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Definition
competes with thymosin b4 for G-actin
binding, favoring polymerization |
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Term
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Definition
In the cell, a large amount of free G-actin is not
available because it is bound by thymosin b4 |
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Term
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Definition
| capping protein of MFs; binds to minus ends of MFs, preventing loss of subunits there |
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Term
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Definition
| capping protein; binds to plus ends of MFs to prevent addition of subunits there |
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Term
| Capping proteins -- what are they? Bonus: give example of two capping proteins and say where they bind. |
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Definition
Capping proteins bind to the ends of a
filament to prevent further loss or addition of
subunits. Example: CapZ, tropomodulins |
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Term
| Proteins That Sever Actin Filaments -- what do they do? Bonus: give an example of one such protein. What else does this protein do? |
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Definition
• MFs are broken up by proteins that sever and/or
cap them
• They cause the actin gel in the cell cortex to
soften and liquefy |
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Term
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Definition
breaks actin MFs and caps the newly
exposed plus ends, preventing further
polymerization |
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Term
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Definition
| in filopodia; keeps the actin tightly bundled |
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Term
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Definition
-Actin may be bundled into tightly organized arrays,
called focal contacts or focal adhesions used to
connect the cell to the extracellular matrix
-alpha-actinin is a protein that is present in such
structures |
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Term
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Definition
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Term
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Definition
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Term
| Proteins That Bundle Actin Filaments -- what do they do? Bonus: give an example. |
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Definition
• Some actin-containing structures can be highly
ordered
Example: Fascin in filopodia keeps the actin tightly bundled |
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Term
| Microvilli -- what are they and how to they relate to MFs? |
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Definition
-Actin bundles in microvilli are the best-studied
examples of ordered actin structures
-Microvilli are prominent features of intestinal
mucosal cells; they increase the surface area of
the cells in order to absorb more nutrients
-The core of a microvillus consists of a tight
bundle of microfilaments with the plus ends
pointed toward the tip |
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Term
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Definition
| WHAT IS IT; laterally crosslinks actin |
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Term
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Definition
| WHAT IS IT; laterally crosslinks actin |
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Term
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Definition
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Term
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Definition
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Term
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Definition
• Often, actin forms as loose network of
crosslinked filaments
• One of the proteins important in the formation
of these networks is filamin
• Filamin splices/joins the two MFs together
where they intersect |
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Term
| Proteins That Link Actin to Membranes -- why is this necessary? Name one such protein. |
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Definition
• MFs are indirectly connected to the plasma
membrane and exert force on it during cell
movement or cytokinesis
• This indirect connection to the membrane
requires one or more linking proteins
• Example: erythrocyte membrane has band
4.1, spectrin and ankyrin proteins |
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Term
| Besides loose networks and bundles, what is another organizational form of actin? |
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Definition
Besides loose networks and bundles, actin can
form dendritic (treelike) networks (also found in
lamellipodia) |
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Term
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Definition
• Besides loose networks and bundles, actin can
form dendritic (treelike) networks (also found in
lamellipodia)
• A complex of actin-related proteins, the Arp2/3
complex, nucleates new branches on the sides of
existing microfilaments
• Arp2/3 branching is activated by a family of
proteins that includes WASP (Wiskott-Aldrich
syndrome protein) and WAVE/Scar |
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Term
| WASP (Wiskott-Aldrich syndrome protein) and WAVE/Scar |
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Definition
• A complex of actin-related proteins, the Arp2/3
complex, nucleates new branches on the sides of
existing microfilaments
• Arp2/3 branching is activated by a family of
proteins that includes WASP (Wiskott-Aldrich
syndrome protein) and WAVE/Scar |
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Term
| What regulates when/where actin-based structures assemble? Example of some actin-related proteins? |
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Definition
-Cell signaling regulates when and where actin-based structures assemble
• Both plasma membrane lipids and several
small G proteins related to Ras regulate the
formation, stability, and breakdown of MFs |
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Term
| Phosphatidylinositol-4,5-bisphosphate (PIP2) -- how does it relate to MFs? |
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Definition
Phosphatidylinositol-4,5-bisphosphate (PIP2) can
bind to profilin, CapZ, and ezrin (MF anchoring
protein)
• PIP2 recruits these proteins to the cell membrane
and regulates their interactions with actin
• CapZ binds tightly to PIP2 resulting in its removal
from the end of a MF, promoting disassembly of MF
• Gelsolin interacts with a different phospholipid in a
similar manner, resulting in uncapping of MFs |
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Term
| Rho GTPases -- what do they do? Some key Gho GTPases? |
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Definition
• Three key family members are RhoA, Rac1, and Cdc42
• The cytoskeleton of cells exposed to certain growth factors can undergo a dramatic change
• Many signals that result in these changes act via a family of monomeric G proteins called Rho GTPases |
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Term
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Definition
• Activation of the RhoA pathway results in formation
of stress fibers (by activating formin, which helps
form long, unbranched MFs) |
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Term
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Definition
• Activation of the RhoA pathway results in formation
of stress fibers (by activating formin, which helps
form long, unbranched MFs)
• RhoA also regulates assembly of the contractile ring
during cytokinesis |
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Term
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Definition
• Rac1 activation results in extension of lamellipodia
(involves WAVE protein and Arp2/3 complex) |
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Term
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Definition
Cdc42 activation results in the formation of
filopodia (involves WASP and Arp2/3 complex) |
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Term
| Regulation of Rho GTPases |
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Definition
NEED TO LEARN THIS BETTER --
GEF "activates" Rho pathway by adding ATP; GAP "deactivates" pathway by removing phosphate and returning to GTP |
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Term
| Rho family GTPases -- how are they affected by cancer? |
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Definition
• Rho GTPases play an important role in cancer:
they are required for cell motility that leads to
invasion and metastasis
• These genes are rarely mutated but are often
overexpressed in cancers
• GAPs, GEFs and GDIs are also known to be
affected in cancer cells |
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Term
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Definition
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Definition
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Definition
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