Protein Sorting II: Moving Proteins into Endomembrane System (L15) Flashcards
common pathway for protein sorting
- protein synthesis and translocation across ER membrane
- protein folding and modifications in the lumen of ER
- protein transport to Golgi, lysosomes, or cell surface in vesicles
how is trafficking done?
w/ vesicle packages - 50-100 nm in diameter that pinch off one compartment, then dock, fuse and drop off materials in next compartment
what is the default pathway for packages?
secretion onto plasma membrane
other functions of ER
- make steroids
- start point for cholesterol synthesis
- Ca sequestering
- cyt p450 detox
- lipid synthesis
- glycerophospholipid synthesis
- post translational modifications (glycosylation, disulfide formation, Pro or Lys hydroxylation)
how are secreted proteins protected from degradation?
by being internalized in a membrane bound compartment
composition of SRP
ribonucleoprotein complex of 6 proteins and a single 300-nt RNA molecule
where does SRP bind?
- to SP on nascent polypeptide in cytoplasm and stops translation until docked on the ER
- also binds the large ribosomal subunit and the SRP-receptor on the ER
SRP receptor
integral membrane protein, heterodimeric complex with a and B subunits
function of SRP receptor
docks the SRP, ribosome, and nascent polypeptide onto the ER
what is special about the SRP-p54 subunit and SRP receptor a subunit?
together, they can hydrolyze GTP (but cannot on own) to release SRP from ribosome and allow continued translation and translocation into ER
what does BiP stand for? what is it?
binding protein - an Hsc 70 chaperone that uses ATP hydrolysis to draw in the protein
what determines the topology of single-pass proteins?
internal stop-transfer-anchor (STA) and signal-anchor (SA) sequences
when predicting which sides are cytosolic or luminal, where do the positively charged amino acids always stay?
positive residues always face cytosol
what type of plot can help predict the type/class of integral membrane proteins?
hydropathy plots
what does GPI stand for?
glycosylphosphatidylinositol
what are GPI-anchored proteins?
a fifth type of membrane-anchored protein made from type I proteins by luminal transamidase
function of GPI-anchored proteins
role in signaling, cell polarity, or interactions w/ ECM
what post-translational protein modifications occur in the ER?
- proteolytic cleavages
- GPI-anchors
- N-linked glycosylation
- disulfide bond formation
- hydroxylation of Pro and Lys residues
functions of ER
- post-translational protein modifications
- ensures proper protein folding
- assembly of multiprotein complexes
- targets misfolded proteins for degradation
describe N-linked glycosylation of proteins in ER
- occurs on a subset of Asn residues
- adds on core of (GlcNAc)2(Man)3 always
- commonly adds on (Glc)3(Man)9(GlcNAc)2
how is the mannose sugar tree for N-linked glycosylation
on a dolichol lipid anchor on the ER membrane
- add a phosphate +GlcNAc to dolichol phosphate
- add another GlcNac
- add 5 Man
- flip to ER lumen
- add 4 more Man
- add 3 glucose
function of N-linked glycosylation
may promote folding and stability of glycoproteins
where does disulfide bond formation occur? what protein does this?
formation and rearrangement occur in rER
catalyzed by protein disulfide isomerase (PDI)
where does hemagglutinin formation occur? what proteins are responsible?
folding and assembly in ER
done by BiP, PDI, calnexin and calreticulin (lectins)
