Lecture 9 - quiz 1 Flashcards
how can cellular membranes be made
by expanding pre existing membranes not de novo
describe how all proteins encoded
by nuclear genes and translated in cytosol
few exceptions (mito)
when must proteins be sorted
during or after translation to their correct compartment or membrane
sorted by signals
where is sorting info stored
inside proteins themselves
in sequence of proteins - determines where, ptms and folding
how are proteins transported across pore
if folded = hard to be transported across pore
usually co translationally = before has folded
many ribosomes attached to er membrane
if folded = hard to move through
describe what happens when secretory pathway proteins are inserted into or across er membrane
transport to further compartments - golgi, pm, enodosmes, lysosomes
outer and inner nuclear membranes are continuous with er
describe rough er
many attached ribosomes
secretory protein synthesis
describe smooth er
no ribosomes
sites of lipid synthesis
describe targeting signals
sequences within a protein that specify its organelle location - zip code or signal peptide - so cell reads and is like you go here
often independent from structure or biochemical function of proteins
recognized by their pattern but not usually an exact sequence, independent of region with hydrophobic patches
when are targeting signals removed
may be removed by proteolysis after targeting complete or form part of native structure
describe targeting steps - 3
- recognize a signal on a protein
- connect protein to the membrane
- translocate protein in to or across the membrane
describe signal hypothesis
secretory proteins were known to enter er during translation of their mrnas
describe signal hypothesis observation
a newly translated secretory protein is larger than its final form
describe signal hypothesis hypothesis
extra sequence is targeting signal peptide whose main function is to direct insertion into er
signal peptide must start mechanism to connect ribosome to translocation pore
signal peptide is cleaved off after targeting is finished
describe describe signal hypothesis experimental approach
if has ribosomes - rna encoding for secretory pathway = then dissociated
if has er too = now nascent peptide recognized by pore = pushed into er
describe ribosome exit tunnel
Nascent polypeptides exit the ribosome through a tunnel in the large (60s)
subunit
Tunnel is neutral, polar, too small for tertiary folding
Surface around exit site provides binding sites for ER targeting mechanisms
30 to 40 amino acids of nascent polypeptide between peptidyl- transferase site and the exit
describe how secretory signal peptides direct proteins
to er for translocation into or across membrane
co translationally mostly
what do many secretory pathway proteins have
additional targeting sequences that direct them to organelles
often a polypeptide sequence pattern - motif
sometimes a ptm
describe organelles not in secretory pathway
have their own targeting signals
describe signal peptide pattern
hydrophobic central region 8 or more residues long with short polar regions on each side
where are signal peptides in most cases
at N terminus = do not want protein to fold in cytosol
shorter hydrophobic regions - 8-16 residues
often cleaves off after translocation
describe signal anchors
signal peptides that also become tm helices
not cleaved off
can be in diff places in protein
longer hydrophobic region = 18-24 residues
describe signal peptide –> targeting steps - step 1
- recognize a signal on a newly translated protein= ribosome begins translating polypeptide with a signal, Signal Recognition Particle (SRP) is a soluble protein that binds signal and ribosome during translation
describe signal peptide –> targeting steps - step 2
- connect protein to the membrane= SRP Receptor (SRP-R) is a membrane protein that binds the ribosome-SRP complex, SRP-R links ribosome to translocon pore in ER
