Chapter 17 Flashcards
Define myocyte
a long, tubular cell that develops from myoblasts to form muscles
How is skeletal muscle formed? or Why are skeletal muscle cells special?
Formed by the fusion of many individual myoblasts to form a unique multinucleated cell
What is the name of the parts of muscle fibers? What structures do they contain and how are they arranged?
Myofibrils
Repeating structural units called sarcomeres that extend from Z “discs” or Z “lines”
Thick filaments make up the center of the sarcomere, and thin filaments are attached to the Z discs on either side of the thick filaments
What are the subparts of sarcomeres made of?
Thick filaments are made of many type II myosin and thin filaments are made of actin
Give the major tenet of the sliding filament model of contraction
the mechanism by which muscle contracts depends on the contraction of individual sarcomere units
Describe the sliding filament model of contraction
the type II myosin that makes up thick filaments have their heads bound to the actin (thin) filaments and their tails bound to each other
the myosin heads use ATP to walk along the actin filaments towards the Z disc, bringing the two Z discs closer and creating a contraction
the length of the thick and thin filaments does not change between the contracted and relaxed orientations, they simply slide relative to each other
In what direction does myosin move in muscle contractions? Describe a full cycle of the changes in myosin conformation.
towards the plus end of the actin filaments
tightly bound to AF → not bound to either
release of AF → bound to ATP
cocking of head → bound to ADP/P
powerstroke → ADP
tightly bound to AF → ADP released, not bound to either
What two structures surround each myofibril? What is their purpose?
sarcoplasmic reticulum → stores calcium
T tubules → infoldings of the muscle fiber plasma membrane, conduct signals
What is the neurotransmitter that starts a muscle contraction?
acetylcholine
Describe the process of the muscular neurotransmitter being converted into a release of ions that regulate muscle contractions. Include channels (3).
action potential travels along the axon until it gets to the neuromuscular junction → resulting in a change in the membrane potential of the T tubules → which cause Ca+2 to be released from sarcoplasmic reticulums
voltage-gated Ca channel → T tubule, transmits signal to sarcoplasmic reticulum
calcium-triggered Ca channel → SR, triggers a release of Ca in response to Ca from T tubule
Ca/ATP pump → SR, pump Ca back in
How does calcium regulate muscle contractions?
the actin (thin) filaments are composed of actin proteins as well as tropomyosin and a troponin complex
in the absence of Ca, the myosin-binding sites are blocked by tropomyosin
binding of Ca to troponin opens the binding site by moving tropomyosin
What are some effects of constant activation of the muscle contraction signaling pathway?
muscle rigidity, sharp drop in ATP levels, large increase in heat
Describe the size of intermediate filaments
between that of microtubules and microfilaments (actin)
Describe the protein components of intermediate filaments
can be made of various types, each will have fibrous or filamentous elongated proteins with two globular heads at each end
Describe the organization of molecules into intermediate filaments
monomers twist to form dimers, dimers form tetramers, eight tetramers form a ropelike filament
associated with each other by non-covalent bonds
middle section has an alpha helical structure
Describe the polarity of intermediate filaments
non-polar
Describe any nucleotides associated with intermediate filaments
does not bind either ATP or GTP
What cell regions of attachment are intermediate filaments associated with?
desmosomes (cell-cell) and hemidesmosoems (cell-ecm)
What are the four types of intermediate filaments?
nuclear → nuclear lamins in all animal cells
keratins → epithelia
vimentin → connective tissue, muscle cells, glial cells
neurofilaments → nerve cells
Describe the structure and the function of the nuclear lamina
meshwork or lattice-like structure of intermediate filaments underneath the nuclear envelope
provides structural support to nuclear envelope, attachment to chromatin, phosphorylation initiates nuclear envelope breakdown in mitosis
What is the name of the major microtubule organizing center and what are its subparts?
centrosomes
two centrioles at right angles to each other, centrosome matrix of proteinous pericentriolar materia y-tubulin ring that stimulates MT polyermization by acting as a template and nucleating growth
What is the name of the major micotubule organizing center in cilia and flagella?
basal bodies
Describe the dynamic instability of microtubules
always growing and shrinking, some MTs are shrinking at the same time others are growing
Describe the organization of molecules into microtubules
heterodimer of alpha and beta tubulin (both bind GTP, only beta hydrolyzes), which meet end-to-end to form a protofiliment, 13 of which form the microtubule
Describe the polarity of microtubules
the end with the alpha subunit is “negative” and the end with the beta subunit is “positive”
MTs can grow and shorten at both ends, but the plus end is more dynamic due to lower required concentration of free heterodimers
Describe the three phases of MT kinetics
Lag Phase/Nucleation → dimers form into protofilaments
Elongation Phase → rapidly grow through the addition of heterodimers to the plus end
Plateau Phase → off and on rates are about the same and the MT stays the same length
What does MAP stand for?
microtubule associated proteins, or proteins that bind to MTs
What are the three types of MAPs and what do they do?
Bundling Proteins → bind to two sections of MT
+TIP → bind to the plus end of a MT and promotes growth through stabilization of the GTP cap
Catastrophins → bind to the plus end of a MT and promote depolymerization, involved in mitosis
What is tau?
a MAP bundling protein that binds microtubules in axons and helps stabilize them
What are the three main functions of cytoplasmic microtubules?
Structure and Organization → internal skeleton, organize Golgi and ER, polarize cells
Intercellular Movement → tracks for vesicles and mitochondria
Separate chromosomes during mitosis and position the cytokinetic furrow
What drives the movement of flagella and cilia?
Ciliary dynein is attached by its tail to one MT, and attached by its two heads to an adjacent MT
walking towards the minus end produces bending due to cross-linking proteins
Describe the organization of the cytoskeletal structure of cilia and flagella
MTs organized into a 9+2 array
+2 are central pair
9 doublets made of one whole and one partial MT
Proteins link the 9 to the 2 and link the 9 together
Describe the structure and function of the GTP cap in MTs
many tubulins bound to GTP on the plus end of a MT, more are added faster than GTP is hydrolyzed
b/c GTP-tubulin bind more tightly to their neighbors, the GTP cap stabilizes the MT
when the cap is lost, depolymerization is rapid
What do the MT motors kinesin and dynein have in common?
both have a tail and two globular ATP binding heads
use energy from ATP hydrolysis to move in a walking motion, with one head bound at a time, carry cargo such as vesicles and organelles (mitochondria, Golgi, ER)
What are the two ways kinesin is different from dynein?
it moves towards the plus end, while dynein moves towards the minus end
kinesins are processive, which means they can catalyze consecutive reactions without releasing their substrate
Where are the centromeres, ER, and Golgi located relative to each other and which motors are used for transport between them and to the plasma membrane?
centromeres are located next to the nucleus, between the golgi (closer to plasma membrane) and the ER
ER → Golgi = towards minus end, uses dynein
Golgi → ER = towards plus end, used kinesin
Golgi → Cell Membrane = towards plus end, uses kinesin
Cell Membrane → Golgi = towards minus end, uses dynein
Describe the organization of molecules into actin filaments
actin monomers bind ATP → hydrolyzation provides energy for filament formation → each monomer binds to two others to from an actin filament
Describe the polarity of actin filaments
plus and negative ends have different chemical and dynamic properties
polymerization occurs on the plus end (addition of monomers) and disassembly/de-polymerization occurs on the minus end (ATP hydrolysis)
Describe the three phases of actin filament polymerization
Lag Phase or Nucleation → polymerization is very slow, rate limiting step is formation of trimers
Elongation/Polymerization → filament is rapidly becoming longer
Steady State → plateau where most subunits are in the filament and the off and on rates are equal
Describe the function of nucleating proteins in actin filaments
increase the rate of actin filament formation by decreasing or eliminating the lag phase
Describe the function of monomer-sequestering protein in actin filaments and give the names of a few of them
regulate the organization and binding of actin filaments by decreasing the rate of assembly
thymosin and profilin bind to monomers to prevent them from joining the filament
What are the two types of proteins that have the opposite effect of monomer-sequestering proteins
formins and actin-related proteins
Describe the function of bundling proteins in actin filaments
bundle proteins together to increase stability/power
What is the difference between type I myosin and type II myosin?
type I mainly moves vesicles
type II are involved in muscle contraction, cytokinesis, and other types of cell motility
Describe the function and structure of the ARP complex. Where are they found?
(arp=actin related protein)
dense meshwork of actin in a branched structure with plus ends pointing out
made of ARP2, ARP3, and G-actin that forms a trimer that decreases the lag phase
binds to the side of actin and promote branching
Describe the function and structure of microvilli
rigid and non-motile structures that increase the surface area of the plasma membrane, aiding in uptake and secretion of nutrients
parallel actin bundles provide support
How do actin filaments function in cell movement?
Polymerization of actin filaments creates force that pushes against the plasma membrane
Contraction (of back end) requires type II myosin
Describe lamellipodia
Broad
Branching filaments
Plus end towards the plasma membrane
Describe filopodia
Spiky
Parallel filaments
Plus end towards the plasma membrane
Describe the cytoskeletal structure of contractile bundles
Parallel
Heading in both directions
What are formins?
a nucleating protein that attaches to the growing plus ends of actin filaments and promotes the addition of new monomers to form straight, unbranched filaments
What is the cell cortex?
Actin dense regions just under the plasma membrane that supports the cell
What is the Rho family and what do they do?
GTPases that transmit signals from outside the cell to inside the cell can can cause the formation of stress fibers or the ARP complex
