The Cytoskeleton: Microfilaments

what are microfilaments?

actin filaments found in virtually all eukaryotic cells (smallest of the filamentous cytoskeletal elements)

what are the general functions of microfilaments?

muscle contraction, cytoplasmic streaming, furrowing in animal cell cytokinesis, and making motility possible in non-muscle cells

what make up microfilaments?

G-actin monomers

G-actin monomers come together to form what?

actin polymers with a helical structure

how does Listeria monocytogenes, a gram + saprophytic bacterium living in the soil that causes Listeria disease, have motility?

expresses Act A protein that promotes G-actin polymerization that propels it forward to adjacent host cells

actin microfilaments exhibit what?

molecular polarity

what end of the microfilament are actin monomers rapidly added to?

(+) barbed end

what end of the microfilament are actin monomers rapidly lost from?

(-) pointed end

initially, each G-actin monomer binds what?

ATP

when does hydrolysis of ATP into ADP occur?

after incorporation into microfilament, but it is not a required step

muscle cells/fibers contain numerous myofibrils which are subdivided into what?

sarcomeres

what are sarcomeres?

repeating contractile units

what does each sarcomere consist of?

thin actin microfilaments and thick myosin filaments

an elongated protein that fits into the actin helix groove to prevent myosin binding

tropomyosin

a regulatory trimeric protein with 3 subunits

troponin

what troponin subunit binds tropomyosin

TnT

what troponin subunit binds actin microfilament

TnI

what troponin subunit binds calcium

TnC

what is the motor protein that converts chemical energy into mechanical force?

myosin

myosin II is associated with what?

muscle tissues

sarcomeres have a banded appearance: what are characteristics of A bands?

dark bands: actin microfilaments + myosin thick filaments

sarcomeres have a banded appearance: what are characteristics of I bands?

light bands: actin microfilaments

sarcomeres have an H zones, what are it’s characteristics?

light region in the middle of the A band where myosin tails come together

sarcomeres have a Z line, what is it?

end of microfilaments linked together

what nerve cells are associated with muscle cells?

neuromuscular junctions

what is released by neuromuscular junctions that causes an action potential to be generated over the muscle cell surface?

acetylcholine

infoldings of the muscle cell plasma membrane whose purpose is to connect the outer muscle fiber to the interior cell membrane, thus allowing the action potential generated from acetylcholine to penetrate the interior where it causes Ca2+ gated channels to open in the sarcoplasmic reticulum

T-tubules

what causes tropomyosin to move toward the center of the actin microfilament groove, exposing myosin binding sites by pulling the TnT subunit and tropomyosin to the side?

binding of cytosolic Ca2+ to the TnC troponin subunit

what occurs when muscle cells no longer receive a signal?

Ca2+ ion channels close, Ca2+ is pumped back into the sarcoplasmic reticulum, and it dissociates from TnC and reverts back to original position with tropomyosin obscuring myosin binding sites

what cell secretes and organizes most of the extracellular matrix in most animals and people?

fibroblast

what are the 4 steps that describe fibroblast movement?

lamellipodium extension, formation of new attachments, forward movement of cell constituents, and breaking of attachments along trailing edge

when does lamellipodium extension occur?

when Arp 2/3 complex nucleates actin monomer polymerization

what allows cellular constituents to move forward?

tension on the actin cytoskeleton

at what angle and shape does the lamellipodium extend?

70 degrees, fan shape

in summary, what do microfilaments do?

generate tension across the cytoskeleton in conjunction with myosin