Flashcards in exam 3b Lecture 29 Microtubules Deck (56):
In what cells are intermediate filaments (IFs) abundant and diverse?
What is the role of intermediate filaments? Describe filament systems.
They are flexible and strong and thus provide mechanical support for cells and tissues. Filament systems are extensive and link from one cell to another.
With what other cytoskeletal component do intermediate filaments align?
What is the polarity of intermediate filaments?
They are nonpolar.
What do IF proteins not bind? How do they behave in cells?
They don’t bind or hydrolyze nucleotides. They can be dynamic in cells.
Structure of IF proteins
They have globular head and tail domains. They have coiled-coil domain form in middle.
What do IF monomers have in middle?
An alpha helical region
How are IFs constructed?
An IF monomer pairs with another monomer and forms a dimer. The conserved central rod domains are aligned in parallel and wound together into a coiled-coil. Two of these dimers then line up side by side to form an antiparallel tetramer of four polypeptide chains. Within each tetramer, the two dimers are offset with respect to one another, thereby allowing it to associate with another tetramer. In the final 10-nm ropelike filament, tetramers are packed together in a helical array.
How many dimers are in the final ropelike filament? Where do the dimers point?
16 dimers (32 coiled-coils). Half of these dimers are pointing in each direction.
What are the soluble subunits of intermediate filaments?
Dimers and tetramers.
Do dimers have structural polarity? Do tetramers?
Dimers have structural polarity. Tetramers don’t.
How is assembly of ropelike filaments controlled?
By phosphorylation of globular domains.
What are four types of IF?
Nuclear, vimentin-like, epithelial (cytokeratins) and axonal (neurofilaments)
Nuclear IF polypeptides
Lamins: A-type, B-type
Type I keratins (acidic) and Type II keratins (basic)
Nuclear cell type
Vimentin-like cell type
Mesenchymal cells, connective tissue, muscle cells
Epithelial cell types
Epithelial cells and derivatives (hair, nails)
Axonal cell types
Where did intermediate filaments primarily evolve?
What domain do all IFs have?
Which is the most ancient intermediate filament protein?
What are nuclear lamins part of?
The “nucleoskeleton” of invertebrate and vertebrate metazoans
What are the two types of nuclear lamins?
A-type and B-type lamins
Where are B-type lamins expressed? What are they essential for?
In all cells. Essential for viability.
Where are A-type lamins expressed? What do mutations result in?
Expressed in differentiated cells and not essential, but mutations result in diseases affecting muscle, neurons, premature aging
What parts of cell do nuclear lamins interact with?
Chromatin, inner nuclear membrane, nuclear pore proteins
Describe Lamin B structure
Three domains: head, rod and tail. Rod has four subunits, 1a, 1b, 2a, 2b, from head to tail. Tail has NLS and Ig protein, and farnesyl group for association with nuclear membrane.
Why is farnesyl group added to tail of Lamin B?
For association with nuclear membrane.
What controls Lamin B filament assembly?
Phosphorylation site between head and rod domains.
What is the nuclear lamina?
A network of intermediate filaments assembled from lamin proteins. Part of the nuclear envelope.
What does nuclear lamina interact with?
SUN domain proteins that cross inner nuclear membrane.
What do SUN domain proteins interact with?
KASH-domain proteins that span outer nuclear membrane and bind components of the cytoskeleton.
What is LINC system? Where is it found?
SUN domain and KASH domain proteins. It is found in many cell types including plant cells.
What does LINC system interact with?
Actin cytoskeleton, microtubules, and centrosome
What are the functions of lamin? (4)
a. structural support for the nucleus, anchor sites for chromatin b. role in DNA replication, Pol II transcription, mitosis c. assembly/disassembly of the nuclear lamina during the cell cycle and d. lamin B provides a “matrix”, deposited by Ran GTP, for assembly of the mitotic spindle.
What is progeria?
Premature aging caused by Lamin A mutation.
Do lamins have a homolog?
No, but in plant cells, there is a family of proteins like this intermediate filament family.
How does assembly/disassembly of nuclear lamina work during the cell cycle?
Start at interphase nucleus with intact lamina. From interphase to prophase, lamins are phosphorylated and nuclear envelope fragments with divided chromosomes. Between prophase and early telophase, lamins are dephosphorylated and nuclear envelope fragments fuse and are fused into two daughter cells at late telophase.
What is the role of neurofilaments in neurons?
They stabilize axons with cross links.
Does tubulin need coiled-coin domain? Why or why not?
No. It is a globular protein that holds GTP. It binds and hydrolyzes GTP.
How do neurofilaments determine axon diameter?
The number of neurofilaments determines diameter.
What is axon made up of?
Microtubules and neurofilaments.
Where are cytokeratins expressed?
In epithelial cells.
What kind of cytokeratins assemble? (2)
Heterodimers of Type I (acidic) and Type II (basic) proteins. They coassemble into four filaments.
How many different genes for cytokeratins do humans have?
What does cytokeratin gene expression depend on?
It is tissue-specific.
What do cytokeratins interact with?
Desmosomes. Connect with basal lamina as hemidesmosomes to provide mechanical support to tissues. Interact with actin and microtubules also.
What do plakin family proteins link IFs to? (2)
Plakin family proteins link IFs to other cytoskeletal filaments. They link IFs to contacts with other cells, extracellular matrix.
What do mutations in cytokeratin genes affect?
Mechanical strength of tissue.
Symptoms of epidermolysis bullosa (2)
Blistering of skin, epithelial tissues in digestive tract
What genes are affected in epidermolysis bullosa?
KRT5, KRT14, cytokeratins; PLEC – plectin; C7 – collagen
What recent therapy has been used to correct epidermolysis bullosa?
Transplant of bone marrow cells