Dissecting the Complexity of Cell Structure 2: Microfilaments. How actin-based polymers contribute to cellular structure, behavior, and motility. An overview of the actin cytoskeleton in a cultured epithelial cell, using antibodies and immuno- fluorescence.
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How actin-based polymers contribute to cellular structure, behavior, and motility
An overview of Microfilaments
Actin is at the cell cortex and also organized into fibers that extend through the cell.
Formation of microfilaments (MFs) from Microfilaments
the protein actin
Actin polymerization is controlled, in part, by an Microfilaments
initiating complex that includes actin-like proteins
The Arp2/3 complex not only initiates MF Microfilaments
polymerization, it can bind to the walls of
existing MFs, promoting the formation of
branches, which turn that region of the cell
into a gel (as opposed to a “sol”).
Actin at the Microfilaments
edge of a
at all actin
(top) or only
at the cell periphery
Actin polymerization membrane-end of MFs
is also regulated by
small proteins that
bind to soluble actin
and modify its
behavior in solution.
Here is profilin, which
catalyzes the exchange
of ADP for ATP in the
actin cleft, increasing
the pool of polymer-
There are also proteins that retard MF growth: membrane-end of MFs
it by making a
complex that will
Diagramatic representation of the pathways membrane-end of MFs
The help to regulate MF formation
Proteins that bind to the walls of MFs can bind membrane-end of MFs
them together in different geometries
Four proteins that bind the sides of MFs and membrane-end of MFs
bundle them into different geometries
Diagrams of examples of MF bundling membrane-end of MFs
TEM image and diagram of MF membrane-end of MFs
Bundling in a microvillus
Diagram of MF web formed when cross-linking membrane-end of MFs
is done by the long, fibrous protein, Filamin
When filamin is under-expressed, due to mutation, cell morphology and motility are abnormal. Cells depleted for Cell expressingfilamin by LOF mutation normal filamin levels
Like MTs, MFs can bind some associated morphology and motility are abnormal.
proteins that alter the properties of the polymer.
Cofilin binds to F-actin and distorts the polymer,
making it less stable.
Gelsolin also bind the MF wall, distorting is and morphology and motility are abnormal.
inducing breaks that shorten the average fiber.
Gelsolin and morphology and motility are abnormal.
proteins like it
can break up MF
either in vitro (as
shown here) or
in vivo, leading to
rapid reshaping of
Breaks in MFs mean both more and shorter morphology and motility are abnormal.
MFs therefore more ends (for a given amount
of polymer). Thus, both growth and shrinkage
of polymer can be faster after MF severing.
Capping proteins, like Cap-Z can silence one morphology and motility are abnormal.
MF end for further subunit addition; in this case
it is the fast-growing (plus) end that is
All these processes can be regulated to make morphology and motility are abnormal.
For a quite complex behavior of the MFs in vivo.
Blood platelets (thrombocytes) looking as they
do in circulating blood (left), after attachment
to a surface (center) and during a platelet
“reaction”, where they attach to a substratum
and contract. All this is MF assembly-dependent.
But What Initiates MF morphology and motility are abnormal.
at the PM?
Projection morphology and other actin-dependent shape changes suggest that controlled polymerization can also give rise to unidirectional fibers, the filopodia
Formins are now recognized as molecules that can initiate MF polymerization at the PM and allow continued addition of actin at the PM while an association with the membrane is maintained
Actin “stress fibers” are initiated by and attached to the cell cortex.Cell margin seen with The same region seen withoptics that show proximity fluorescence optics and anbetween the cell’s surface antibody that lights up actin
and the underlying substratum
These are “focal adhestions” that mark site of cell attachment
Another view of stress fibers (green) and “focal adhesions”, stained with antibodies to vinculin, a component of stress fiber adhesion sites
To promote tissue strength, cadherins are specifically but non-covalently bound to the MFs of the cytoskeleton. The linking proteins include catenins, which play a role in signaling between tissues.
Cadherins bind the MF cytoskeleton of one cell to that of its neighbors, forming a mechanical
unit. This coupling contributes to the mechanical integrity of a tissue.
In some cells, cadherins and actin MFs form ordered arrays that can work like a contractile ring and control the cell’s diameter.
Integrins are membrane proteins that bind cells that can work like a contractile ring and control the cell’s diameter.
to the “Extra-Cellular Matrix” (ECM). The integrins make bonds between the actin cytoskeleton and the fibers of the ECM, such as collagen and fibronectin.
Transmembrane proteins link the actin-dependent cytoplasmic motility that pulls on MTs with extracellular material, such as beads that can attach to the cell’s surface
Thus, we can recognize a mechanical continuum that runs from cytoplasmic MFs, controlled in part by MTs, through the plasma membrane to the molecules of the extracellular matrix (ECM).
Reminder of the mechanism for signaling by small G-proteins, like Rac, Rho, and CDC-42: GTP-bound state is active and turns on down-stream proteins. GAPs and GEFs control the relative concentrations of GTP- and GDP-bound forms of the protein
Discovery of such control pathways has motivated a
Search for homologous paths in other cell types