Cell Motility and Shaperequire microfilaments (F-actin), microtubules and intermediatefilaments.

Not surprisingly, the actin skeleton isdynamic, not just the tracks.

Fibroblasts migrate through tissues by extending forward, attaching to the substrateat focal adhesions and pulling along the tail.

Actin assembly anddisassembly is themechanism.

Actin is highly conserved.

G actin assembles into F actin. ATP holds together the two lobes. Polymerizationcan be done in vitro. ATP is hydrolyzed but this is not required for assembly.F actin depolymerizes back to G actin.

F actin has structural and functional polarity. Can’t be seen directly.Myosin (S1 heads) bind F actin assymetrically.

The actin cytoskeleton is organized into bundles andnetworks to provide framework.

Cortical actin is netlike andcytosolic networks are meshes.

Cortical nets are attached to the membrane

Actin crosslinkers:

Fascin in bundles,Filamin in networks

There are many otheractin crosslinkers, eachwith actin-binding domains.

RBC, network provides flexible structure.

HypotheticalJunctional complex

Know components

Left: Platelets at restMiddle: Induced to spreadRight: Stellate pattern during clot retraction

All are actin based and coupled via ABPs tothe actin cytoskeleton

Actin bundles support projections

Microvilli (epithelial) and filopodia (fibroblasts)

-3 stepsNucl. Is marked by lag phaseElong by addition at both endsSS at the critical concentration

ATP is eventually hydrolyzedso that internal actin is ADP form

Growth is greater at + end

Polar growth is demonstrated bydecorated nuclei

End growth differences due todifferences in Cc

Cc is measured by capping oneend and measuring C for growth

G actin ADP, Cc for bothends are equal

Treadmilling

Tools: cytochalasin D depolymerizesby bind plus ends (fungal alkaloid)Lantrunculin binds G actin and blkspolymerization (sponges)Phalloidin, binds F actin and blksDepolmerization (Death Angel)

Summary

Cell dynamics are controlled by dynamicsin the actin cytoskeleton network

Cell shape changes by filament size and treadmilling

But shape changes can also occur by actin severing proteins