Cytoskeleton (+ intro) Flashcards
why does compartmentation exist?
structural organisation and set up distinct biochemical environments for specific reaction
genome size
bacteria/archaea - size related to number of protein coding genes
eukaryotes - genes and genome size not linear relationship (humans much bigger genome but same no. genes as worm) because of multicellular complexity
how do cells cope with multicellularity?
cell cycle: 4 phases
gap phase prepares for replication and monitores env.
S phase where genome copied
M phase (prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis, and checkpoints in between)
controlled proliferation and PCD (apoptosis/autophagy)
3 types of cytoskeleton (diagrams on word)
microfilaments (actin): 7-9nm diameter
microtubules (alpha-beta dimer): 25nm
intermediate filaments: 10nm
cytoskeleton function
makes sure compartments are in the right place,
vesicles transported
cytoskeleton in epithelial cells in gut
microvili made from actin
microtubules
interfilaments link desmosomes and hemidesmosomes for structural rigidity
microtubule structure (diagram on word)
barrel with lumen forms tube
13 protofilaments round in circle and link together to form barrel
protofilaments are repetitive alphabetaalphabeta tubulin
a-b dimer (homologues of each other) bind existing microtubule
solid, rigid, hard to snap under pressure
2 diff ends: beta +ve end, alpha -ve end
so motor proteins go in 1 direction
2 ends of microtubule
+ve beta end
-ve alpha end
microtubule growth (Cc, GTP) (diagram on word)
alpha-beta tubulin dimers only form polymer when above critical concentration (Cc)
Cc threshold lower at + end (than -ve) so grow at + end
can also grow at ‘-‘ when high enough conc. but preferential ‘+’ growth
increased conc. means increased tubule mass so longer
reducing below Cc means disassemble also at + end
a&b both bind GTP, hydrolysed to GDP on beta to form main body of tubule
+ has faster polymerisation than hydrolysis so GTP not turned to GDP so makes GTP cap at + end
hydrolysis faster at - end so always GDP form
treadmilling (microtubules)
rate of addition at + end = rate of disassembly at - end
so stays same length
taxol drug (microtubules)
binds beta and prevents depolymerisation of microtubule
treatment for ovarian/breast cancer by preventing mitosis
colchicine and colcemid drug (microtubules)
binds alphabeta dimer prevent addition (polymerisation) to microtubule
MTOC (microtubules)
microtubule organising centres - point from which they emerge
conc around nucleus
originate from single point
grow from centrosome
dynamic instability (microtubules)
get to point when stop growing then depolymerise and collapse to spot (MTOC)
centrosome (microtubules)
2 centrioles in centrosome matrix make a centrosome
close to plasma membrane
duplicated every cell cycle
microtubules grow from gamma-tubulin ring complexes (nucleating sites) - bind minus end (a) so prevent addition
accessory protein in gamma is bridge between microtubule and centrosome
shrinking and dynamic instability (microtubules)
growing filaments are straight
shrinking are curved/frayed
hydrolysis to GDP when touch membrane so peel off and back to dimers
conformational change in beta changes angle of dimer so cap lost and fall apart from loss ability to stick
dimers turn to GTP form again so can add
catastrophe then rescue
3 functions of microtubules in mitosis
separate chromatids (+ attaches to centromere) interpolar microtubules in the middle, 2 sitting on top of each other and forces apart
astral microtubule connect to plasma membrane, to centre the spindle in cell so 2 cells are same size
MAPs (microtubules)
microtubule associated proteins regulate MT stability and function
+ end MT binding proteins (microtubules)
EB-1 DASH ring complex cross-linking, stabilising, bundling proteins (bind site of MT) MAP2 Tau MAP65 (Ase1)
EB-1 (microtubules)
end binding protein 1
only binds GTP tubulin on growing MTs so stabilise cap and MT seam (in between filaments, seam closes barrel into tube)
other protein complexes can bind so move to plasma membrane as grows
Dam1/DASH complex (microtubules)
fungal specific heterodecamer
bind polymerising/depolymerising MTs and couples kinetochore movement to MT depolymerisation
proteins bind in ring around tubule - inside of ring -ve and outside of MT -ve so repel each other, don’t touch but glide along protofilaments
splaying MT prevent ring falling off in depolarisation (like fraying to the sides)
MAP65 (microtubules)
bind sides and stabilise anti-parallel MTs
important for bi-polar spindle formation in middle where from diff side on top of each other
MAP2 & Tau (microtubules)
binds sides and stabilise parallel MTs to keep apart and protect (stabilise axon for vesicles transport of NT)
promote polymerisation and inhibit catastrophe
+ve tail binds -ve MT surface
MAP2 tail bigger than Tau tail so gap between MTs bigger when use MAP2
MAP2: dendrites, between MTs and interfilaments
Tau: dendrites/axon, bridge between parallel MTs
Alzheimer’s and microtubules
Tau MT binding protein mutated so can’t transmit vesicles for synapse so defect in brain
motor proteins
use ATP to walk, mostly to +ve end, cargo can be another MT to slide against
Kinesin: stabilise, de-stabilise, bundle MTs, +ve end directed (some -)
Dynein: fast -ve end directed
can have both types for movement in both directions
structure of kinesin (diff types)
head, stalk, tail
kinesin-1 = conventional, homodimer 2 same subunits kinesin-2 = heterodimeric kinesin-5 = bipolar, 2 heads both attach to MT kinesin-13 = speed disassembly
kinesin movement
head binds ATP/ADP for walking
1) motor head has catalytic core (in 2 motor heads) and neck linker (linking 2 catalytic cores)
binds to MT (1 head always bound) and ADP released so ATP enters binding site
2) conformational change so neck linker zips onto catalytic core of 1st head so throws 2nd head forward to next binding site on MT because loses bond on MT
3) behind head now ATP to ADP
dynein structure (see diagram lecture 18 page 2)
2 stalks sit on MT with dynactin (co-factor for dynein)
2 heavy, 2 inter, 2 light chains so double headed and 2 binding domains but requires dynactin
dynactin binds heavy chain stem connected to head domain where ATPase is
6 ATPase domains arranged in wheel including major ATPase
Arp1 (actin related protein) polymer, link dynein to cargo domain
dynein transport
major ATPase causes conformational change so changes tail position relative to wheel and stalk changes where bound to MT
centrioles
make centrosome
MTs arrange in circle
2 MT circles share subunits for 2 centrioles (like infinity shape)
actin overview
double helical twist structure diff types with diff functions binds ATP (not GTP like MTs) formed from large no. diff places so lots in cell highly dynamic grow and shrink at both ends (polarised +ve and -ve end) tracks for myosins motor proteins form static and motile structure
types of actin structure: static and motile (diagrams on word)
arranged in diff way for diff function
microvilli are static cells in monolayer
leading edge: crosslinked filaments force plasma membrane forward for migration
stress fibres: parallel bundles squeeze cell to push nucleus forward for migration
(long lines in middle of cell while leading edge crosslinks on front edge)
contractile ring: when dividing in cytokinesis
cell cortex: inside edges
