Courses/Bioc 406
The function of the signal recognition particle (SRP) is
In O-linked glycoproteins
The synthesis of N-linked oligosaccharides occurs in the ____while O-linked oligosaccharides are synthesized in the ______.
I-cell disease results in psychomotor retardation, skeletal deformities and death. Which of the following is true regarding this disease?
Signal sequences are not required on proteins destined
N-terminus of the growing peptide worms through an exit tunnel in the large subunit
Targeting signals tell the proteins where to go
| Target | Usual signal location | signal removed | SIGNAL |
|---|---|---|---|
| ER | N-terminal, or internal | +/- | |
| Mitochondrial matrix | N-terminal (multiple) | + | |
| Peroxisome | C-terminal | - | |
| Nucleus | Internal | - |
Nuclear localization signals (NLSs) direct proteins to the nucleus
Membrane targeting: cotranslational protein translocation Translocon (preprotein translocase) (Sec61 complex) Protein-conducting channel with an aqueous pore that spans the ER membrane N-terminal hydrophobic signal sequences direct secreted and membrane proteins to the translocase ![[Pasted image 20260303135004.png]] ^+ charge then 6-15 hydrophobic amino acids
SRP (signal recognition particle) ribonucleoprotein A. Recognize signal sequence emerging from ribosome exit tunnel B. pause translation (when binds hydrophobic stretch) C. direct ribosome and nascent polypeptide to preprotein translocase channel on endoplasmic reticulum membrane SRP cycle SR (SRP receptor) binds SRP at ER Hydrolyze GTP, lets ribosome dissociate from SRP Translation resumes because ribosome docks ER, without SRP or SR in the way
Ribosome exit tunnel is highly conserved because it must interact with SRP and translocase
Secretion proteins threading through translocon, pushing and pulling from chaperones signal peptidase cleaves signal peptide
Integral membrane proteins Hydrophobic stop-transfer-peptide- biding site
Important for folding, function, targeting Most proteins synthesized at the rough ER are glycosylated (N-linked glycosylation) Glycosylation signal is Asn-X(Ser or Thr)
N-linked oligosaccharides are added by block transfer in the ER ![[Pasted image 20260303140550.png]]
Sugars are added from nucleotide sugar donors Donor sugar, nucleoside diphosphate + ROH acceptor sugar -> oligosaccharide by glycosyl-transferase
Synthesis of O-linked oligosaccharide ![[Pasted image 20260303140930.png]]
O linked occurs in Golgi Occurs on the OH group of Ser or Thr Occurs on completed polypeptide chains Occurs by the serial addition of single monosaccharide units N linked occurs in ER and Golgi Occurs on the NH2 group of Asn in the N-X-S/T sequence motif Occurs cotranslationally (well, at least the ER part) Occurs by the block transfer of a 14-residue core oligosaccharide in the ER, then trimmed and modified further in the ER and Golgi, but always retains the same 5-residue core Penta saccharide
Functions of glycosylation Recognition, targeting, antigens, hiding antigens, protein folding and structure
I-cell disease Clinical phenotypes Skeletal defects Large intracellular accumulations (inclusions) of fats and complex carbohydrates Lack phosphotransferase needed for addition of M6P to lysosomal proteins in fibroblasts...secreted...
Mannose 6-phosphate (M6P) targets proteins from Golgi to lysosome ![[Pasted image 20260303141949.png]] ER, Golgi, Vesicles, Plasma membrane or Er, Golgi, Vesicles, lysosome, Golgi
Impaired glycogen breakdown in lysosomes
M6P targets lysosomal enzyme to treat