8: Actin in Non-Striated Muscle, Intermediate filaments, and cell adhesion
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What does actin and myosin do in non-skeletal muscles?
• cytokinesis • smooth muscle • vesicle transport • cytoplasmic streaming • cell migration
contractile ring/adherens/stress fibres belts is similar to
sarcomere (rolled up)
what is the difference between sarcomeres and contractile rings/adheren belts
unstable = depolymerizing to shrink the ring in cytokinesis not Ca2+
smooth muscle contraction is
non voluntary + persistent (arteries, moving digestive tract)
How is smooth muscle different from striated muscles?
not coated in troponin and tropomyosin induced slower but persistent bc myosin binding site of actin not blocked
What is the regulator of smooth muscle contraction?
ca2+ (external and released from SR) activates kinase activates myosin regulatory light chains alters myosin conformation via calmodulin (CaM) = contraction
How does vesicle transport work with actin filaments?
• myosin 5 bound vesicles are carried along actin filaments • can transport many types of vesicles and mRNA • actin (+) end leads to the bud and (+) end actin inside the bud with formin for poly.
when is vesicle transport used
budding yeast cells - transport mRNA, organelles, proteins into new bud by myosin walking along cortical actin also organizes Mts
What is cytoplasmic streaming?
• found in large cells (plants) • cytoplasm constantly moves around periphery of the cell for increase diffusion effectivity • cortical actin network filaments surround moving cytoplasm
Where does cell migration occur
embryos
What are the actin structures involved in cell migration?
filopodia: fingerlike projection actin bundles lamelipodia: large branched and networked actin pushing membrane forward by poly stress fibers: bundles of actin connected to substrate/EXM on which it is moving (focal adhesion) by binding to integrins
What are integrins
transmembrane PM proteins that hold stress fibres to connect to extracellular matrix during cell migration
what is chemotaxis?
movement of cells due to high [] of chemoattractant
receptors and g-proteins are found
around entire cell periphery
What are the steps of cell migration/movement?
• focal adhesion (cell stuck to substrate) and between focal adhesions are stress fibers and activate G-proteins on moving side to make actin • lamellipodium extension • new adhesion of lamellipodium + new stress fiber + integrins • translocation by cell body by actin/myosin movement, moving towards new adhesion • deadhesion
proteins involved in cell migration?
Ras superfamily of small GTPases (ie Rho proteins) - Cdc42 (arp2/3) - Rac (arp2/3) - Rho (formin) active in GTP form
In a dominant active Rho cell..
a lot of unbranched microfilaments bc of formin activation = long actin bundles -> stress fibers
In a dominant active Cdc42 cell…
a lot of filapodia made (membrane protrusions)
In a dominant active Rac cell…
membrane “ruffles” and is extending in all directions at once (lamelipodia everywhere)
What is a wound closure assay? What does it study?
scratch confluent cells to see if the cells are migratory if they move into the wound but they must have all of Rac, Cdc42, and Rho
What are the steps of activation of GTPase proteins for cell migration?
• growth factor binds to cell surface receptor which adds GTP to Cdc42 and Rac • Cdc42 activates WASp and Rac -> Arp2/3 -> actin polymerization -> filopodia forms • Cdc42 also activates Par6 -> gives cell polarity • Rac activates WAVE -> Arp2/3 -> actin poly -> lamellipodia • Rac + cell surface protein activates Rho-GTP -> forming activated and RacGTP inactivated -> actin poly -> strip finer forms • Rho kinase activates myosin
adult cells do not
undergo cell migration
Cdc42 important in
polarity AND coordinating actin and microtubules
what does Par6 do?
• gets activated by Par 6 • induces MT growth and kinesis+dynein activation which will help MT bring proteins and monomers for actin formation at direction of polarization