Cell Bio Exam 3

microfilament functions (5)

actin

• supports cell shape

• contractile cell movement (amoeba)

• cytokinesis in cell division

• transports organelles and vesicles

• muscle contraction with myosin

intermediate filament functions (4)

rope-like fibers

• lamins support nuclear shape

• cell-cell junctions (desmosomes and hemidesmosomes)

• tissue integrity (skin, hair, nails)

• mechanical resistance to stress (ex. junctions strengthen tissues)

microtubule functions (5)

• act as scaffold

• transport in the cell via motors

• mitosis and meiosis spindles (chromosome separation)

• structure and movement in flagella/cilia

• organelle positioning

how are actin monomers made

G actin is in the cytosol, SO

• free floating ribosome translates mRNA

  • no signals (NLS, ERSS)

• HSP-70 folds in the cyosol

Actin filament growth and formation

• formed from 2 protofilaments made from G-actin

  • + end fast growing, - slow growing

• only add monomers on the end = POLAR

conditions for G-actin to add to protofilament

ATP!! and salt for ionic bonds

G can only bind if ATP is bound

after addition to F, associations w neighboring G’s causes shape change to hydrolyze ATP→ ADP

why can’t you add G’s to middle of protofilament

• protein-protein interactions between G’s

  • H bonds, ionic bonds

  • each G surrounded by 4 other G’s

Rate limiting step of actin polymerization

• nucleation

  • nucleus is a trimer

  • needs time to spontaneously form from random diffusion of G’s

  • dimer unstable until third G adds

Desmosomes

• disks

• links cells together with cadherins

• links to intermediate filaments

Adherens

• belt shaped all around the cell

• part of it uses cadherin

• links to actin filaments

hemidesmosomes

• disk shaped

• links cells to ECM with integrins

• links to intermediate filaments

which cell-cell junctions use cadherin

desmosomes (disks) and adherens (belt)

Which junction uses integrins

Cell-ECM adhesion (hemidesmosomes and focal adhesions)

What are cadherins

• IM proteins that bind cells together with homophilic interactions

• homophilic: E-cad to E-cad and N-cad to N-cad, N and E don’t bind

• cadherins need Ca+ to extend their arms (n term) and bind toe ach other

how do cadherins mediate cell sorting

homophilic interactions: E and N cadherins only bind to like cadherins, so E separates from N

based on E or N concentrations and needs Ca

Integrin structure

• alpha and beta dimers in many combos

  • alpha has S-S bond

• bind cell to RGD sequences in ECM with Ca++ (Ca needed to extend to reach ECM proteins)

• N term outside, C inside (S-S bonds in the ER, so alpha stays together)

• in hemidesmosomes and focal adhesions

• heterophilic (alpha-beta)

• bind to collagen or fibronectin in the ECM

Fibronectin and integrins

Fibronectin has 2 RGD binding sites per monomer (4 total)

loops so RGD is exposed

integrin binds with Ca++

adherens junctions

Cell-to-cell: attach cells by their actins using cadherin

desmosome structures

disk-shaped, bind adaptor proteins on cytosolic side

actin protofilaments

2 protofilaments, each is a row of G-actin+ADP

twist around each other

Rates of actin polymerization

Kon = # G’s added/total G’s

Koff = # G’s removed/ total G’s

higher total G’s = higher chance

rate = assembly - disassembly

critical concentration

Kon = Koff at critical concentration

treadmilling caused by different CC’s

critical concentration graphs

CC’s are X int’s

+CC lower than -CC (need higher concentration for - end)

2 types of nucleating proteins and which ends they bind

1. Arp23 (branching): binds - end of new F or new branch

2. Formin (linear): binds + end

Arp23 complex

Arp2 and Arp3 bind together in a complex with accessory proteins

mimic + end as a nucleus, caps the - end so won’t decay below -CC

skips nucleation → create an F rn

can bind at 70° on formed F’s for branches

Branched Arp23

branches add more strength and stability

bind at 70°

formins

formin dimer donut + formin whiskers

whiskers have profilin that grabs ATP and G’s

myosin II and V structure

• both two heads

• lever arm (light chains)

  • 2 for II, 6 for V

• alpha-helicial coiled coil tails

  • II longer (bipolar filaments, tails bind together)

• V binds cargo

ADP release of II and V

fast II, non processive

slow V, processive