Chapter 26

Protein Sorting

Chapter Objectives

• Understand the pathways of cotranslational processing of proteins– ER, Golgi, Plasma membrane, Lysosomes

• Understand the pathways of posttranslational processing of proteins– Mitochondria, Peroxisomes, Nucleus

Overview

• Cytoplasmic proteins• Mitochondria,

peroxisome, or nuclear proteins

• Extracellular, lysosomal, ER proteins

Cell Reminder

ER

Golgi

How do proteins know where to go?

• Signal Peptide– Sends

proteins to ER

– No signal, to the cytosol

• Blobel 1999 Nobel Prize

Some polar

Hydrophobic (10-15 aa)

Mature protein

Cleavage site

N

To the ER!

GDPGTP

GDP

Oligosaccharides – N-linked

Goodbye ER, Hello Golgi

• Must physically cross a space.

• COP-coated vesicles– Coat protein

complex• COP-II sends

vesicles to Golgi

Finding a Target

• ARF GTPase activity activated at target– Coat is removed

• Target– Plasma membrane

• Nerve cells

– Other organelles

Oops!! Goodbye Golgi, Hello ER

• COP-I• KDEL sequence in protein

Review

To the Lysosome• Vesicles bud from Golgi to a variety of places– Lysosomes• Termernal glycosylation is mannose 6P• Destructive

Clathrin-coat

• Another way to make vesicles– Dynamin pinches

vesicle• GTP dependent

– Clathrin-coated vesicles become endosomes• Endosomes can

become or fuse with lysosomes.

Integral Membrane Proteins

To the Mitochondria

To the Peroxisome

• Peroxisomes are thought to be artifact organelles– Oxidation reactions• Without producing energy

– Break down lipids

– No synthesis of proteins– PTS1 and PTS2 are signals to import proteins• PTS1 has a c-terminal SKL tripeptide

To the Nucleus

• Transport proteins– Proteins and RNA are leaving the nucleus– Proteins are trying to get into the nucleus– Some proteins are really big

• Nuclear Pore Complex (nucleoporin)– Very big allows diffusion of up to 40,000 Da

proteins– Larger proteins must be accepted by complex

Nuclear Pore Complex

Big Protein Transport

• NLS (nuclear localization signal)– Very positive charge• PKKKRKV• KRxxxxxxxxxxPAAIKKAGQAKKKK

• NES (nuclear export signal)• Some proteins go both ways– Heteroribonucleoprotein complex (hnRNP)• Shuttles mRNA out of nucleus• Returns to pick up more• 38 amino acid signal sends it both directions

Entropy and Transport to Nucleus

• Not energy mechanism innate to nucleoporin

• Uses small GTP/GDP binding protein Ran

Ran as an Energy Mechanism

Controlling Export/Import