microtubules and actin comparison worksheet

microtubules subunit proteins

α, β dimers

actin filament subunit proteins

actin

microtubules diameter

25nm

actin diameter

7nm

microtubules number of protofilaments

13 subunits

actin filaments number of protofilaments

2 filaments

are microtubules hollow or solid

hollow

are actin filaments hollow or solid

helical→ solid

microtubules bind what nucleotide

GTP/GDP

actin filaments bind what nucleotide

ATP/ADP

microtubules polarity

alpha- minus, beta- plus

actin polarity

positive- barbed, negative- pointed

microtubules nucleation factor

gamma tubulin

actin filaments nucleation factors

arp 2/3- branched (70 degrees angle)

formins- linear initiation

microtubules cellular location of nucleation

centrosome-MTOC

actin filaments cellular location of nucleation

wherever- mostly cell cortex and plasma membrane

microtubules function

transport of vesicles

organelle positioning

mitotic spindle

actin filaments function

mechanical support

scaffold for signalling

cargo transport

cell motility

muscle contraction

dyanmic instability

microtubules will disassemble rapidly

GTP→ GDP hydrolysis makes it change conformation when phosphate leaves→ catastrophe (rapid dissassociation)

treadmilling

actin- elongation of positive end, shortening in minus end

elongation/ shortening happen at similar rates- generates more force in the positive end→ movement

ADF/ cofilin

actin disassembly

profilin

actin assembly

capping protein

actin capping protein at + (barbed) end→ stops treadmilling

microtubules post translational modifications

acetylation- more resilient to breakage

glutamylation- recognized by enzymes to cut it

nucleation factor role

reduces the concentration of polymers req for assembly

Why are microtubules ideal for positioning organelles?

- microtubules are large→ making them very rigid

- hollow structure resists compression

- form long tracks spanning the entire cell (high rigidity over long distances)

Why is actin ideal for generating contractile force?

- actin generates tension

- actin is thinner and more flexible

- it forms dense networks and interacts with other proteins to generate tension

- rapid localized force generation