生物化學原理課件（英文）：Chapter40 Post-translational processing and protein targeting and sorting

1、Chapter40 Post-translational processing & targeting/sorting Outline Post-translational processing 1.Cleavage of polypeptides 2.Addition of aa to N-terminal 3.Splicing of protein 4.Chemical modification of individual aa 5.Binding of cofactors 6.Protein oligomerization 7.Protein folding Targeting and

2、sorting of proteins 1.Signal hypothesis 2.Co-translational targeting & sorting 3.Post-translational targeting & sorting Post-Translational Processing During translation, about 30-40 polypeptide residues are relatively protected by the ribosome. Once the polypeptide chain emerges from the ribosome it

3、 starts to fold and can be subject to post-translational modifications. Why post-translational processing? 1) adds functionality 2) effects targeting 3) regulates activity 4) increases mechanical strength 5) changes recognition Cleavage of Prepro-insulin Covalent modification Acetylation, Lipidation

4、, Amidation, Disulfide cross-linking, Phosphorylation, Glycosylation Methylation Sulfation, Vitamin K-Dependent Modifications( -carboxylation) , Vitamin C-Dependent Modifications (hydroxylation) Ubiquitination and sumoylation Formation of Correct Disulfide Bonds Protein Splicing Protein splicing is

5、defined as the excision of an intervening sequence (the Intein) from a protein precursor and the concomitant ligation of the flanking protein fragments (the Extein) to form a mature host protein and the free intein Inteins as Mobile Genetic Elements: Homing Endonuclease Activity Examples: Totals:48

6、different species and strains. 7 eukaryotes (3 in unicellular organisms, 3 in plastids and 1 in a viruse),24 bacteria (3 in bacterio- and pro- phages),17 archaea. 128 inteins 236 types of protein family hosts 5. 111 separate proteins with inteins:94 with a single intein, 2 pairs each with one split

7、intein 2, 12 with 2 inteins and 3 with 3 Mechanism- a self-catalyzed process Intein-mediated protein splicing occurs post-translationally Mechanism of Protein Splicing Protein Folding Primary structure determines 3D struture Proper and fast Folding of most proteins needs to be assisted by molecular

8、chaperones-HSP70 & Chaperonins Folding of some proteins requires protein disulfide isomerase (PDI) Folding of some proteins involves peptide prolyl cis- trans isomerase (PPI) Protein Sorting & Targeting Signal peptide hypothesis ( Proposed by Gnter Blobel - 1999 Nobel Laureate in Physiology or Medic

9、ine ) : Proteins have intrinsic signals that govern their transport and localization in the cell. These signals are special aa sequences, which are like the zip codes or address tags. In the absence of signal peptides, a protein will remain in the cytoplasm Detailed mechanism: four common features 1

10、. Proteins are made as preproteins containing signal peptides 2. Membranes involved in protein translocation have specific receptors on their cytosolic faces 3. Translocases catalyze the movement of the proteins across the membrane with metabolic energy (ATP, GTP, ion gradients) essential 4. Preprot

11、eins bind to chaperones to stay loosely folded Protein Sorting & Targeting Protein Sorting & Targeting Three-step process 1.Recognize 2.translocate 3.mature Two examples 1.co-translational targeting :ER, Golgi, lysosomes, plasma membrane, secreted proteins 2.post-translational targeting: nucleus, mi

12、tochondria, chloroplasts, peroxisomes Mechanism of Protein Sorting & Targeting Sorting Signals in Eukaryotic Proteins Sorting Signals in Eukaryotic Proteins Translocation of eukaryotic proteins into the lumen of the ER Signal recognition particle (SRP) EBrings protein to ER EBinds signal sequence on protein EB