Week 5 - ER translocation + protein folding

Signal Recognition particle (SRP)

guides protein with the signal sequence to the ER

SRP receptor

place where the SRP binds to in the ER membrane

ER translocation channel

the signal sequence is bound to this channel to open it up, while the rest of the protein is passed through the channel

signal peptidase

enzyme that cleaves off the signal sequence from the rest of the protein - the signal sequence is then released and soon degraded

N terminal signal sequence

initiates translocation

stop transfer sequence

halts translocation

transmembrane proteins

formed from hydrophobic regions of protein that anchor themselves in the ER membrane (ie: signal sequence and stop transfer), while the others pass through the membrane

Muli-pass proteins are embedded into the ER membrane in a…

sewing machine like fashion

heat shock proteins

• provide protection against many stresses

• highly conserved

• protein chaperones, disaggregates, proteases, etc.

molecular chaperones

HSP-70; monomeric - ATP driven…

chaperonin

HSP-60; multimeric, barrel protein in which misfolded proteins can enter in order to fold correctly

Retrotranslocation

process of tagging misfolded proteins (right as it is released into the cytosol) for proteosome to degrade

ways to manage accumulation of misfolded proteins

• increase chaperone expression

• upregulation of ubiquitin proteosome pathway

• degrade transcripts

• decrease translation

• expand ER membrane

• reduce ER to Golgi trafficiking

• ER associated degredation

• apoptosis

N-linked glycosylation

glucose is added to where asparagine amino acids are located (on portions that are inside the ER lumen)

how does the glycosylation help guide proteins through the ER?

an unfolded protein undergoes continuous cycles of glucose trimming (by glucosidase) and glucose addition (by glucosyl transferase), maintaining an affinity for calnexin and calreticulin until it has achieved its fully folded state

endoH

protein that recognizes and cleaves the sugar modifications on proteins after they have entered the ER

microsomes

fragmented ER portions that are only found and used in test tubes and experiments

without microsomes……

there is no membrane to translocate across

protein is accessible to proteases

will not be N-linked glycosylated

if endoH is added but no proteins are cut….

the protein is in the cytosol

Unfolded Protein Pathway

occurs within the ER

Heat shock response

hsp 60 and 70 help to refold proteins