Protein Targeting Flashcards
two classes of polyribosomes and what classifies them
- classified based on location, they are exactly the same chemically
- cytosolic ribosomes and RER associated ribosomes
Cytosolic ribosomes produce proteins that end up in
cytosol
mitochondria
nucleus
peroxisomes
RER associated polyribosomes create proteins that end up in
outside of the cell
plasma mem
lysosome
RER, golgi, and vesicular mem proteins
Signal peptide
location on protein
job
- located on the N terminus of the protein, the end which is first synthesized (the 5’ codon of the mRNA)
- targets nascent proteins and polyribosomes to the RER
absence of a signal protein
the protein is synthesized in the cytosol by default
steps involved intargetting the RER for protein translation
- ribosome binds to the mRNA in the cytoplasm and translation begins
- the first 10-20 aa’s constitute the signal peptide
- a signal recognition particle SRP binds to the signal peptide of the nascent protein and stops translation
- SRP binds to a specific SRP receptor on the RER. THe SRP, still bound to the nascent protein and ribosome, move to the RER
- a channel forms in the membrane
- translation begins again with the protein being threaded through the channel into the RER lumen
- SRP is released and recycled
- as the protein emerges into the lumen signal peptidases cleave off the signal peptide
- proteins to be secreted are deposited in the lumen where they acquire additional folding.
- membrane proteins conatin an internal hydrophobic region which stops translocation on the prtion across the the RER mem, anchoring the protein in the lumen
post translational targeting from the RER default plasma mem ER mem lysosome
- default: protein is secreted
- plasma mem: will contain at least one stop/anchor sequence, embedding it in the Er mem
- ER mem: special signal sequence called the KDEL sequence tells the cell to retain the protein in the ER
- lysosome: shuttled to the golgi where modification will occur, creating a high mannose N-glycosylated glycoprotein
targetting of lysosomal proteins
- shuttled to the golgi which is where modification will occur
- they recieve a phospho-mannose group which act as the code that targets them for the lysosome
- targetting is then partially mediated by a receptor in golgi for the phospho mannose group
packaging of proteins destined for lysosomes after modification
- the mannose-6-phosphate signal sequence binds with an M6P receptor in the trans-golgi
- this receptor ligand complex will bud off of the trans golgi as a clathrin coated vesicle
- clathrin will be lost as it moves through the cytoplasm, simultaneously it will be acidified
- the receptors will be pinched off in a vesicle and sent back to the golgi and the protein will be carried in a transport vesicle to fuse with the lysosome.
Targetting of
mitochondrial
nucleus
peroxisome
- mitochondrial: have N-terminal signal eptide sequences that facilitate entry into the mitochondrion. secondary amino acid sequences determine where it will end up within the mitochondria
- nucleus: amino acid sequences subsequent to translation allow the protein access to the nucleus via nuclear pores rather than by crossing the memrane
- peroxisomes: the tripeptide S-K-L found near the C-terminus is responsible for targeting
chaperon proteins
- complex with nascent polypeptides and help direct them to the proper location
- importins and exportins are examples
- also help fold newly synthesized proteins into their proper shape; if the protein is incorrectly folded, it becomes a target for the ubiquitin pathway (defective protein destruction system)
- found in many cellular compartments including the cytoplasm, ER, and mitochondria
- many belong to the heat shock protein family. this is because they are promoted under conditions which cause unfolding
lysosomal storage diseases
characterization
cause
research significance
-characterized by the accumulation of biopolymers in the lysosome
-this is caused by disfunctional lysosomal enzymes that can not break down the products in the lysosome
lead to the discovery of lysosomal enzymes as well as the elucidation of the degradation pathways involved
I-cell disease
- characterization
- cause
- inclusion cell disease
- characterized by the accumulation of a variety of biopolymers in the lysosome and a highly elevated secretion from the cell of lysosomal enzymes involved in their degradation
- study of this disease led to the discovery of the phosphomannose code (for targetting the lysosome) and its pathway
- this disease is caused by lack/defectiveness of N-acetylglucosamine phsphotransferase. This causes lysosomal enzymes not to bet tagged with mannoset6P and not make it to the lysosome. They instead get lost in the golgi and secreted. There are NO enzymes in the lysosome in this scenario
Tay-Sach’s disease:
-cause
-results
symptoms
- result of an enzyme deficiency in hexosaminidase A that disrupts sphingolipid breakdown in lysosomes
- results in the build up of GM2 gangliosides
- causes mental retardation, blindness, and early death
Mucopolysaccharidoses 1-4
These are the most common lysosomal storage diseases
-lysosomes become engorged with muccopolysaccharides, now termed glycosaminoglycans
