intermediate filaments and septins

intermediate filaments

strong rope-like polymers of fibrous proteins that resist stretch

how big are IFs?

\~10 nm in diameter (intermediate between actin thin filaments and myosin thick filaments)

are IFs stable or unstable?

very stable – disassembly in vitro requires treatment with agents that denature proteins

do IFs bind nucleotides?

Do not bind nucleotides.

are IFs polar or non polar?

non polar in structure

can IFs withstand stretching forces?

yes

what roles do IFs play in cells?

play a structural or tension-bearing role in the cell and are important for cell strength and cell-cell attachments

IF structure

central αhelical coiled-coil rod domain that is highly conserved and heads and tails that are variable in length and sequence. form a parallel dimer that assembles into an antiparallel tetramer. **Because the tetrameric subunit has no polarity, the filament has no polarity**

Assembly of IFs in cells

Soluble tetramers can be isolated from cells. direct, random exchange between the pool of IF subunits and those in the insoluble filaments of the cell; the dynamics of these filaments are very modest compared to actin filaments and microtubules

tetramers

basic building block of the IF

Intermediate Filament Assembly and Turnover

IF turnover appears to involve a combination of severing and annealing whole segments, tetramer exchange with mature filaments, and tetramer exchange with unincorporated segments.

where do tubulin subunits incorporate?

at growing + ends

Type I (acidic) and Type II (basic) Keratins

form %%heteropolymers composed of acidic and basic subunits%%. There are over 20 keratins expressed in human epithelial cells. Keratins are found in epithelia that line internal body cavities. Because different cell types express different sets of keratins, the ==origin of epithelial cancers (carcinomas) can be determined based on the set of keratins found in the tumor cells==. There are also about 10 ‘hard’ keratins that are found in hair and nails. Keratins in the outermost layer of the skin become cross-linked together, and as cells die the keratins persist as a protective layer.

Type III Vimentin and Vimentin-related filaments

proteins can form %%homopolymers that consist of a single IF protein species.%% Vimentin is the most widely distributed intermediate filament protein. It is found in fibroblasts, endothelial cells, white blood cells, and embryonic cells. Another type III protein, desmin, is found mainly in ^^muscle cells^^ where it links adjacent Z-disks of myofibrils. ==Glial fibrillary acidic protein (GFAP) is a vimentin-related protein found in the glial cells that surround neurons and astrocytes.==

Type IV Neurofilaments

These are composed of %%heteropolymers%% of NF-L, NF-M, and NF-H. They are ==found in axons and are important for providing strength in the axon and determining its diameter==

nuclear lamins

The **nuclear lamina** is a meshwork of IFs that lines the inner surface of the nuclear membrane. It is composed of lamins A, B and C. Nuclear lamins differ from other IF proteins in that they: have ==a longer central rod domain, contain a nuclear import signal, assemble into a two dimensional sheet-like lattice, and are more dynamic than other IFs.== __Disassembly of the nuclear lamina during mitosis is mediated by phosphorylation__.

Intermediate filament binding proteins (IFBPs)

cross-link IFs into networks or bundles. The network of IFs in the cell is often coincident with the microtubule cytoskeleton. In some cell types, disruption of the microtubule cytoskeleton with microtubule de-stabilizing drugs causes the IF network to collapse

Nuclear positioning and support of the nuclear envelope

%%vimentin and keratin%% can extend from the nuclear envelope to the plasma membrane, perhaps ^^helping to position the nucleus.^^ The network of lamins at the inner surface of the nuclear membrane helps maintain the structure of the nucleus and is important for nuclear assembly and disassembly

Cell-cell attachments: desmosomes and hemi-desmosomes

The epithelia of organs and skin are held together by cell junctions called desmosomes and hemi-desmosomes. **Desmosomes** function in cell-cell interactions, whereas **hemi-desmosomes** are important for the interaction of cells with the extracellular matrix. __Intermediate filaments run between desmosomes, hemidesmosomes and the nucleus. Together IFs and desmosomes provide each cell and the entire epithelium with strength and rigidity__

Muscle cell integrity

Desmin links the Z-disks of skeletal muscle together within the myofibril. Moreover, it connects Z-disks to cell junctions in cardiac muscle.

plectin

crosslinks MTs and IFs

Intermediate Filaments and Disease

Genes encoding IFs are associated with >40 clinical disorders

epidermolysis bullosa simplex (EBS)

caused by mutations in keratin genes expressed in skin. The skin of people with this disease is very sensitive to injury

More than 50 different mutations in type-A lamins cause disease

Emerey-Dreifuss muscular dystrophy (EDMD), cardiomyopathy, and progeria (premature aging)

what does Hutchinson-Gilford Progeria Syndrome (Progeria) result from?

the premature aging disease is caused by a mutation to the lamin A gene

septin filaments

10nm in diameter, bind and hydrolyze GTP, and can be found associated with actin filaments and membranes

cells with mutations in septin genes

all arrest cytokinesis

what happens with septin filaments in mammalian cells?

colocalize with F-actin in stress fibers and with F-actin in the contractile ring

structure of septin filaments

GTP binding domain near N-terminus (not sure its role in GTP hydrolysis of filament assembly) and a alpha helical coiled coil near its C terminus. non polar subunits that can assemble into filaments, rings, or cage like structures

Cellular Functions of Septin Filaments: Cytokinesis

In mammalian cells and in budding yeast, septins are found at the cleavage furrow and cytokinetic ring. Septin mutations in yeast cause cytokinesis to fail.

Cellular Functions of Septin Filaments: Scaffolding

Septins may play a role in recruiting signaling proteins such as protein kinases and GTPases that regulate cytokinesis and other aspects of cell function.

Cellular Functions of Septin Filaments: diffusion barrier

Septins can promote compartmentalization of cellular domains.

Cellular Functions of Septin Filaments: membrane trafficking and dynamics

Septins may play a role in the movement and fusion of membrane vesicles and organelle remodeling