lec 10 ER

all proteins begin synthesis in? and what 4 places they end up?

on ribosomes in cytosol

end up:

-in cytoplasm

-in an organelle

-in plasma membrane (or ER or golgi)

what 2 locations in cell are POLYPEPTIDES SYNTHESIZED? create table what proteins synthesized in each

1. RER (rough ER) (1/3 proteins synthesized here)

-secreted, transmembrane, soluble proteins (in er, golgi, lysosomes, endosomes, vesicles, vacuoles

2. Free CYTOSOLIC ribosomes

-proteins that stay in cytosol, peripheral proteins, proteins going to nucleus, proteins in peroxisome, chloroplasts, mitochondria

what are the 2 pathways of protein sorting?

1. Co-Translational Import (during translation)

-protein has ER SIGNAL SEQUENCE that directs ribosome to RER

-soluble (means protein goes into lumen, unline memb proteins stop in memb) protein enter ER while ribosome still making protein, and translation completed on RER (then sorted in ER n golgi)

2. Post-translational import

-proteins LACKING er signal seq complete synthesis on free ribosomes

-if protein dont have sorting signal, remain in cytoplasm

4 important points to consider for protein targeting (also answer how do proteins know where to go like where is that info)

-sorting SIGNAL (info coded in PRIMARY SEQUENCE tell where to go)

-RECEPTOR (for signal seq)

-nature of translocation signal

-ENERGY SOURCE

list the organelles in the endomembrane system and briefly describe what function goes on there (draw it out)

-Rough/Smooth ER + Golgi: sites for protein/lipid SYNTHESIS, PROCESSING/SORTING

-Endosomes CARRY/sort material brought INTO CELL

-Lysosomes DIGEST material/unneeded cellular components

-Transition/Transport/Secretory Vesicles MOVE molecules BETWEEN compartments/plasma membrane

where is the START of the endomembrane system? what does it consist of and whats the internal space called?

ENDOPLASMIC RETICULUM = start

-consists of cisternae (tubular memb + flat sacs)

-LUMEN = internal space of ER

what does the ER signal hypothesis propose?

-intrinsic moleculecular SIGNALS determine localizations of some proteins (if signal deleted, targeting of potein lost)

how many amino acids in ER signal sequence? what does the ER signal sequrnce do? whats one common thing all signals have

15-30 aa in er signal sequence, which is on proteins in co-translational pathway that direct the complex (mrna, polypep, ribosome) to the RER

-common: N-terminus usually has 6-12 hydrophobic aa core

what is the name of the particle that pauses translation in cotranslation? what is bind to and what structure does it lead the complex to

SRP (signal recognition particle)(contains protein/RNA) BINDS to signal sequence on protein to block translation, and also BINDS TO RIBOSOME and directs ribosome to structure in ER membrane called translocon

what is SRP receptor and what does it do?

SRP receptor: Integral Membrane Protein

-on cytoplasmic side of ER MEMBRANE

-complex binds to this receptor, then the ER signal sequnce is inserted into the translocon (channel protein in memb) once SRP released

what triggers the oppening of channel to ER lumen? as polypeptide elongates and go into ER lumen, what happens?

-contact between ER signal sequence with interior of TRANSLOCON displace plug = open channel to lumen

-in lumen, a Signal peptidase CLEAVES ER signal seq (then the seq is quickly degraded)

-Chaperones BIND the peptide (to facilitate folding)

what happens once polypeptide synthesis is complete

-polypeptide RELEASED into LUMEN and ribosome DETACHES from ER membrane

-subunits dissociate + RELEASE mRNA

what are G proteins and compare GTPase vs GDPase

-G proteins (GTPases) act as SWITCHES

-switch between 2 confirmations: GTP- and GDP- bound state (diff affinities for ligands in diff conformation)

GTPase = ON

GDPase = OFF

SRP and SRP receptors are ___ proteins. in which conformation do they have high affinity for eachother? what causes release of SRP

SRP and SRP receptors are G proteins

-in GTP (on), HIGH AFFINITY for eachother

-HYDROLYSIS of GTP changes their conformation + REDUCE AFFINITY = RELEASE

other than soluble proteins, what other major group of polypeptides is SYTHESIZED on RER (co-trans)

molecules that will become Integral membrane proteins (inside memb)

what 2 ways can transmembrane proteins pass membrane? what side of membrane is oligosaccharide chains on? what reflects function in protein snthesis?

-single or multipass

-oligo on NON cyto side (inside)

-asymetry reflects function

some hydrophobic transmembrane proteins DONT HAVE cleavable ER signal sequence, what do they have instead

INTERNAL hydrophobic signal anchor sequence

what has charge asymetry in cotrans of transmembrae proteins and why

Translocon and Membrane have charge asymmetry

(more NEGATIVE on cyto/outside because of more PS)

what decides orientation of MULTI-PASS integral memb protein?

determined by FIRST transmembrane domain