BI220 - Protein Processing and Sorting

where do newly synthesized proteins go

• stay in cytosol

• PM

• organelles

• secreted

signaling sequence

15-60 aa sequence found in protein that directs protein to proper cellular location

• often removed once protein sorted

why are signaling sequences necessary

proteins must have a specific signal sequence to move to specific loc

why are signaling sequences sufficient

addition of specific signal sequence is enough to move protein to specific loc

free ribosome found

found in cytoplasm

bound ribosome found

attached to ER

eukaryotic translation beginning vs end

• all start at free ribosomes

• end depend on protein destination

after polypeptide synthesis completion in cytosol (free ribosome)

• protein dissolves

• protein functions

• imported into non-EMP organelle

  • mitochondria, chloroplast, peroxisome

• import into the nucleus

after polypeptide synthesis initiation in cytosol (bound ribosome)

• ribosome and protein brought to ER

• protein synthesis completed in ER

• protein ends up in EMS organelle or secreted

mechanisms of protein sorting

• nuclear localization

• chloroplast/mitochondrial localization

• ER localization

  • ENP organelle

  • extracellular matrix/secretion

  • PM

nuclear sorting in steps (5)

• completion of protein synthesis in cytosol

• recog nuclear localization sequence (NLS) by importin

• importin transport NLS protein through nuclear pore complex

• RAN-GTP bind to importin

  • release NLS protein

• RAN-GTP recycle importin, GTP hydrolyzed, importin released

nuclear sorting out steps (5)

• completion of protein function in nucleus

• recog nuclear export sequence (NES)

• exportin and RAN-GTP transport NES protein through nuclear pore complex

• hydrolysis of RAN-GTP → RAN-GDP release exportin NES protein

• RAN-GEF swap GDP for GTP to activate RAN

mitochondrial and chloroplast protein sorting steps (5)

• completion of protein synthesis and partial folding in cytosol

• signal sequence recog by receptor protein on outer membrane

• protein translocators align inner and out membranes to thread protein through

• chaperone protein refold protein in matrix

• signal sequence cleaved

main difference between nuclear sorting and mitochondrial/chloroplast sorting

• protein stay folded for nuclear sorting

• protein unfolded and then refolded for mitochondrial/chloroplast sorting

ribosome and protein transport to ER steps - secreted (4)

• signal recog particle (SRP) recognizes and binds to ER signaling sequences of newly forming peptide

  • halts translation

• SRP is ligand for SRP receptor on ER membrane

  • recruits ribosome to ER

• SRP receptor helps bring ribosome to translocation channel protein

  • translation continue in ER lumen

  • signal sequence cleaved

• protein ends up in lumen of ER

  • stay

  • transported to another organelle

  • secreted

ribosome and protein transport to ER steps - bound (4)

• synthesis begins the same

• protein w/ signal-anchor sequence threaded through channel in ER until reached

• ER channel release protein into membrane

• when translation complete, protein stay in membrane

modification to proteins in ER

• protein folding

• attach carbs

• leave via transport vesicles

what happens to misfolded proteins in ER

• ER associated degradation (ERAD)

• unfolded protein response (UPR)

  • ER receptors hold mis-folds which trigger increase in chaperone proteins and decrease synthesis

Golgi

• site of mod on glycoproteins

  • during transit from cis → trans

• tag proteins for final destination

  • via vesicle transport

how do transport vesicles move

• not random

  • guided by microtubules

  • motor proteins: kinesin and dynen

    • use ATP to walk

  • receptor proteins on motors allow for binding to specific vesicle

destination after Golgi

• embedded in PM

• EMP organelles

• secreted

draw decision tree