Cytosol and Cytoskeleton Part 1

What is cytosol?

The intracellular compartment located between the plasma membrane and the organelles/nucleus

What percentage of cytosol is water, and at what pH?

70% water, at pH 7.0-7.4

Name the major ions found in cytosol

K+, Na+, Cl-, Mg2+, Ca2+, HCO3-

What small organic molecules are found in the cytosol?

Amino acids, carbohydrates, lipids, nucleic acids, and inositol phosphates

What types of proteins are found in the cytosol?

Structural, enzymes, signaling, and degradation proteins

List the main functions of the cytosol

Signal transduction, mRNA translation/protein biosynthesis, biochemical reactions (pentose phosphate pathway, glycolysis, gluconeogenesis), proteasomal degradation, and cytoskeleton

Who created the famous illustration depicting the cytosol as a crowded, densely packed environment?

David Goodsell

What are the three cytoskeletal components?

Microfilaments, microtubules, and intermediate filaments

Besides the three filament types, what else makes up the cytoskeleton?

Accessory and regulatory proteins of each filament type

List the functions of the cytoskeleton

Maintenance of cell shape, whole cell movement, contraction, changes in cell shape, structural integrity of the cell, and organization/movement of organelles

What is the monomer form of actin called?

G-actin (globular protein)

What is the polymer form of actin called?

F-actin (filamentous chain)

What are the two ends of an actin filament called?

A growing (plus) end and a disassembling (minus) end

What is required for actin to polymerize?

ATP and magnesium

What stabilizes a microfilament (F-actin)?

Capping proteins at both ends

What is the diameter of thin (actin) filaments?

8 nm

Name the six isoforms of actin and their locations

α-skeletal (skeletal muscle), α-cardiac (cardiac muscle), α-vascular (vascular smooth muscle), γ-enteric (visceral smooth muscle), β-cytoplasmic (non-muscle cells), γ-cytoplasmic (non-muscle cells)

What proteins cap the plus and minus ends of actin thin filaments in muscle?

Cap Z (plus end) and tropomodulin (minus end)

What proteins run along the thin filament to regulate contraction?

Tropomyosin and the troponin complex

What anchors thin filaments at the Z disk?

Z disk proteins Cap Z and α-actinin

What is the motor protein that forms thick filaments in muscle?

Myosin II

Describe the structure of Myosin II

It has a motor (head) domain and a coiled-coil tail; thick filaments are bundled myosin II molecules with a central bare zone

What is the region of a myosin thick filament with no heads called?

The central bare zone (~160 nm)

What are the four steps of the actin-myosin ATPase cross-bridge cycle?

1) ATP binds myosin head 2) ATP is hydrolyzed to ADP+Pi, both remain bound 3) Pi is released, allowing tight actin binding 4) Conformational change (power stroke) releases ADP, resetting the cycle

During sarcomere contraction, what happens to the I band and H band?

They shorten as thin filaments slide over thick filaments (sliding filament model)

What ion triggers muscle contraction and what does it bind?

Calcium, which binds to TnC of the troponin complex

Describe calcium's effect on tropomyosin position

Without Ca2+, tropomyosin blocks the myosin-binding site on actin (OFF); with Ca2+ bound to troponin, tropomyosin shifts to expose the site (ON)

What structure separates cardiac muscle cells from one another?

Intercalated disks

What are the components of an intercalated disk?

Fasciae adherens, desmosomes (macula adherens), and gap junctions

What connects adjacent smooth muscle cells at the membrane dense areas?

Intermediate filaments and mechanical junctions coupling the cells

What are dense bodies in smooth muscle analogous to?

Z disks in skeletal/cardiac muscle (they anchor thin filaments)

In smooth muscle, what activates myosin light chain kinase?

The Ca2+-calmodulin complex

What happens to myosin light chains when smooth muscle is activated?

They become phosphorylated, exposing the actin-binding site and allowing contraction

What enzyme reverses smooth muscle contraction by dephosphorylating myosin light chains?

Phosphatase

Name two actin-dependent processes in non-muscle cells

Cell division (contractile ring) and membrane attachment (focal adhesions)

What protein complex links actin filaments to integrins at focal adhesions?

Vinculin and talin (adapter complex), connecting actin/α-actinin to integrin heterodimers

What do integrin heterodimers bind to on the extracellular side?

Matrix proteins

Give an example of actin filaments determining cell shape via bundled parallel arrays

Microvilli (bundled actin cores) and stress fibers (contractile bundles)

What is the spectrin membrane cytoskeleton also known as (in red blood cells)?

The terminal web / membrane skeleton

Name the members of the spectrin gene superfamily

Spectrin I (erythrocytes only), Spectrin II (all other cells), α-actinin (muscle), dystrophin (skeletal muscle)

What proteins link spectrin to the plasma membrane in red blood cells?

Ankyrin and adducin, along with the anion transport channel (band 3)

What is the function of microvilli?

They increase plasma membrane surface area to enhance absorption and secretion of molecules

What proteins bundle actin filaments within microvilli?

Villin and fimbrin

What proteins anchor microvillar actin bundles to the side walls of the plasma membrane?

Myosin I and calmodulin

What links adjacent actin bundles to each other and to intermediate filaments in the terminal web?

Nonerythroid spectrin (spectrin II) and myosin II

What structure at the leading edge of a migrating cell is rich in actin?

The lamellipodium

Describe the basic cycle of actin-based cell migration

Lamellipodium extends at the leading edge, new adhesions form, the cell body moves forward, and the trailing edge/tail retracts by releasing old adhesions

Approximately how many myosin genes exist?

More than 40

Which myosins move cargo toward the plus end of actin filaments?

Myosin I and Myosin V

Which myosin moves cargo toward the minus end of actin filaments?

Myosin VI

What is the step size of Myosin V movement along actin?

36 nm

What complex links Myosin V to recycling endosome cargo?

RAB11 and RAB11-FIP2