Term 2 Lecture 7: The Cytoskeleton - Intermediate Filaments Flashcards
Types of filaments
Cytoplasmic filaments (keratin)
Nuclear filaments (lamins) form nuclear envelope and provide structural integrity
Both provide mechanical properties specifically strength
Structure of intermediate filaments
-Rod-shaped proteins form strong ‘cables’ throughout the cell
- Short N terminus and globular C terminus domain with alpha-helical domains in-between
-All have 4 alpha helices 1A,1B,2A,2B that form coiled coil structures held together by hydrophobic interactions ( you never see monomers)
-Filaments are 10-12nm thick
-Usually 60-80 kilo daltons
-Actin filaments are 6-7nm And microtubules are ~25nm hence these filaments are called intermediate as they are between these 2 thicknesses
- 2 dimers come together to form a tetramer which is symmetrical and non-polarised
- they associate laterally at each end forming a larger structure, symmetry is maintained and there is no polarity
- no treadmilling in IFs - they are more stable than microtubules and actin filaments
- localised disassembly/reassembly is driven by phosphorylation of dimers
- associate laterally and end to end to form ‘unit length filaments’ and longer filaments
- difference between IF assembly and actin or microtubule dynamics is that there’s no polarity and no involvement of nucleotide hydrolysis assembly/disassembly by phosphorylation
E.g. nuclear lamina when nuclear envelope breaks down for cell division is driven by phosphorylation of individual lamin proteins and it completely disassembles
Many types of IF have a lot of variability in length and C&N termini properties allowing them to form many structures in cells
Epithelial cells have acidic (class 1) and basic (class 2) keratins that provide strength and integrity - the IF network links cells for strength.
Muscle, glial and mesenchymal cells have class 3 proteins: Desmin, GFAP and vimentin for sarcomere organisation and integrity
Neurons have class 4 proteins: neurofilaments (NFL, NFM, NFH) for axon organisation (strength & stabilisation)
Nuclei have class 5 proteins: lamins for nuclear structure and organisation
About 70 IF genes exist in the human genome, over 75 genetic diseases are caused by deficiency or mutation affecting IF filament structure & function
Lamins - gene with the most no. Of phenotypic disease in inherited genetic disorders
IF keratin filaments that join epithelial cells together provide continuity via desmosome links. Inheritable skin blistering diseases result from mutation in particular IF filaments especially keratins e.g. keratin 14 mutant causes detachment of dermis from epidermis and loss of structural integrity.
Summary of IF filaments
A diverse family of proteins that form stable filaments that can bundle
Dimer is the basic building block
Self assemble into antiparallel filaments with no polarity or motors.
Have a structural role
Mutations lead to tissue weakness and degredation diseases
IFs are more stable than microtubules but are also regularly remodelled
IFs stabilise cell structures in reading resistance to stress such as osmosis, mechanical or heat
Cell adhesion
Required for multicellular organisms
Variable strength and duration of adhesion
Often strength in numbers
Cell junction functions
Tight- prevents diffusion between cells
Adherens- link actin cytoskeletons of adjacent cells, mechanotransduction (sends mechanical signals between cells)
Gap- provides aqueous channels between cells e.g. Ca moves through these
Desmosomes- link IF of adjacent cells for tissue mechanical strength
Hemidesmosomes- anchor cells to connective tissue via IFs
Adherens junctions
Found in many stable tissues
Linked in intracellular actin
Also associated with initial contact between cells during tissue formation (connect cells)
E.g. extrusion of apoptotic cells triggered by loss of adherens junctions and contraction of actin ring - bringing surrounding cells together to form new adherens junctions ( so there is no gap where dead cell was previously)
Composition:
Transmembrane proteins: (classical) cadherins, and calcium dependent adhesion molecules that connect to actin cytoskeleton in cell. The e-cadherin is the ‘velcro’ between cells. Adherens junctions link to actin-myosin bundles via branched actin filaments - interactions are dynamic.
These transmembrane proteins are synthesised in the ER
Classical cadherins form Ca dependent adhesion in ECM.
The connection is flexible - would break if rigid.
Millions of these protein chains work together in a velcro-like effect forming a strong and flexible connection that can stretch
Calcium dependent binding between E cadherin protein molecules, millions of these associations form strong flexible bonds.
Cell membrane itself is weak with almost no physical strength so if junctions were connected only to membranes it wouldn’t hold them together.
So in adherens junctions beta catenin proteins bind to E-cadherin that in turn binds to alpha catenin which has an actin binding site - Arp 2/3 disrupts adherens junctions.
Adherens junctions link to actin-myosin bundles via branched actin filaments
Desmosomes
Link IF network of one cell to adjacent cells
Formed of:
Cytoplasmic plaques (plaqaglobin, desmoplakins, plakophilins)
IF filaments bound to the plaques
Linked by desmosomal cadherins (desmoglein and desmocollin)
Strong cell-cell adhesion is mediated by desmosomal cadherins. Desmosomes connect to IFs via plaques.
Desmosomes in disease
-skin blistering diseases e.g. pemphigus vulgaris - an autoimmune disease related to the body’s immune system producing antibodies that attack the desmosome cadherin desmoglein leading to skin blistering
-Mice without desmosomal adaptor protein desmoplakin die during early embryonic development as it is essential for tissue formation
- some rare inherited heart conditions are caused by desmoplakin mutations
IF filament arrangements
Spoke-like arrangement around cell ideal for absorbing tension
Tight junctions
Form a seal between cells
Prevent movement of most water soluble molecules
Restrict movement of membrane lipids and proteins in polarised cells from basolateral to apical surfaces
Histopathological term: zonula occludens
They regulate trans-epithelial transport - paracellular pathway for small molecules and ions between cells is prevented by tight junctions.
E.g. prevents nutrients in the intestines passing between epithelial cells
Restricts movement of membrane lipids and proteins from lumen to bloodstream
This for example prevents gut enzymes entering the blood stream.
Tight junctions have short ECM with strong interactions bringing adjacent cell membranes into close contact.
Claudin and occludin are tight junction proteins. C- terminal tail binds to adaptor proteins such as zo-1 which links tight junctions to actin cytoskeleton stabilising it in particular places
Different tight junctions form tighter/looser adhesions.
