cell molc lesson 9

rough ER

ribosomes outside of membrane → site of protein synthesis

co-translational translocation

process where proteins are transported across or into the ER membrane while they are being synthesized by ribosomes

co-translational translocation steps

1. MRNA binds small ribosome subunit, large ribosome unit binds, translation occurs

2. signal sequence recognized once translated

3. SRP binds signal and ribosome, slowing polypeptide translation → guides whole complex to SRP receptor

4. SRP receptor guides complex towards translocon (peptide channel)

5. SRP/SRPr hydrolyze into GTP → release SRP → translation now continues

6. newly synthesized protein into ER

translocation of soluble proteins

proteins for secretion/retention in lumen of ER

type 1 signal transmembrane proteins

ER bilayer, C-terminus in cytoplasm, N-terminal in ER, hydrophobic

smooth ER

no ribosomes, lipid synthesis, and calcium storage/release

topology of lipids and proteins

cytosolic side always remain cytosolic, luminal side becomes extracellular (established during ER synthesis)

structure of golgi

stacked cisternae, cis face receives from ER, trans face sends to destinations

golgi function

post-translational modifications, sorts/target proteins, synthesizes lipids (sphingomyelin and glycosphingolipids)

lysosomal hydrolases

tagged in golgi with M6P, activated in lysosome, low pH (4-5) maintained by ATPase proton pump

M6P

key targeting signal for lysosomal enzymes that are destined for transport to lysosomes 

UPS steps

E1 requires ATP (E2 and E3 do not), ub binds E1 covalently → E2 → E3 → substrate protein → bind protein → degrade

UPS (ubiquitin proteasome system)

primary mechanism for regulated protein degradation in eukaryotic cells, responsible for degrading short-lived, misfolded, or damaged proteins

secretory pathway

ER → golgi → plasma membrane/lysosome (secretion and organelle biogenesis)

endocytic pathway

plasma membrane → early endosome → late endosome → lysosome (uptake from extracellular space)

vesicle transport step 1

budding: coat proteins induce formation of vesicle, bind cargo and SNARES (includes dynamin)

vesicle transport step 2

movement: vesicles move via cytoskeleton or diffusion (requires ATP)

vesicle transport step 3

tethering: vesicle recognized by target membrane (involves Rab GTPase and tethering proteins)

vesicle transport step 4

fusion: mediated by SNARE proteins, membranes merge

secretory constitutive pathway

secretion is continuous and unregulated

secretory regulated pathway

secretion is directed by hormonal and neural signal (our neurotransmitters)

coat proteins

COP II (ER → golgi), COP I (golgi → ER), clathrin (golgi to plasma membrane)

clathrin structure

3 heavy chain + 3 light chains = triskelion structure

dynamin function

hydrolyses GTP → provide energy to break off vesicle

Rab GTPase active state

binds to GTP → binds to effector (tethering) proteins → helps vesicle go to right target 

Rab GTPase inactive state

Can be converted to active through GEF (guanine nucleotide exchange factor)