Proteins must be targeted to the correct organelle to perform their functions. There are two main pathways for protein targeting: co-translational and post-translational. In co-translational targeting, proteins destined for the endoplasmic reticulum have signal sequences that target them during synthesis via the signal recognition particle and translocon complex. In post-translational targeting, proteins are synthesized and targeted after synthesis via targeting sequences and receptor proteins to their destinations like the nucleus, mitochondria, and peroxisomes. Chaperones assist in protein folding and translocation complexes facilitate transport across membranes.

1. PROTEIN
TARGETING
G AARATHILFA
II M.Sc. ADVANCED BIOCHEMISTRY

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3. PROTEIN TARGETING
• Proteins must be correctly Localized to perform proper function
• Receptors for plasma protein
• DNA polymerase in nucleus
• Catalase in peroxisome
• And insulin outside
• All proteins begin to be synthesized on cytosolic ribosomes .

4. • Sorting or translocation can occur
@ co – translational
@ post – translational
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5.  PROTEIN  Cytosolic function  Synthesis(free ribosomes)
peptides released (into cytosol)
PROTEIN  Nucleus / Mitochondria / Peroxisome  synthesis
(cytosolic ribosomes)  peptides released (cytosol; to be sorted
later/ post translationally)
PROTEIN cell/ membrane nascent peptide targeted to ER
(becomes rough)  sorting (co translationally)

6. # POST TRANSLATIONAL TARGETING
. Cytoplasm
. Nucleus
. Mitochondria
. Peroxisome
# CO TRANSLATIONAL TARGETING (SECRETORY PATHWAY)
. ER
. Golgi
. Lysosomes
. Plasma Membrane

7. +

8. SIGNAL PEPTIDE
• short peptide (16 – 30 amino acid long)
• can be cleaved later by signal
peptidase or remain as a permanent
part of protein
• Can be located on N-, C- terminus or in
the middle of the protein
Mostly N- terminus for secretory
pathway.

11. MITOCHONDRIAL TARGETING

12. # Most mitochondrial proteins are
encoded by nuclear DNA
# Only few are encoded by
mitochondrial DNA and synthesized on
mitochondrial ribosomes
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13. Mitochondrial targeting signal
Usually located at N- terminus of precursor polypeptide
Generally removed in mitochondrial matrix

14. RECEPTOR/ TRANSLOCATION CHANNEL IN Mt
• TOM – translocase of the outer
mitochondrial membrane
• TIM – translocase of inner
memberane

15. • mitochondrial protein are synthesised in cytosol as precursors
• bind to cytosolic chaperone (HSP 70) to keep them unfolded until
they are ready to be translocated
• energy from ATP

18. COMPARISION OF Mt AND CHLOROPLAST

19. PEROXISOMES TARGETING

20. PEROXISOMES
• Single membrane organelle
• Matrix contains oxidative enzymes
lipid oxidation without ATP production
• Proteins encoded by nuclear DNA (everything is imported)

21. TRANSPORT INTO PEROXISOME
• Proteins are synthesized and fully folded in cytosol
• Completely functional and completely folded protein is transported
• This import requires ATP hydrolysis

22. PEROXISOME TARGETING SEQUENCES:
 PTS1 on C-terminus (Ser-Lys-Leu)
 PTS2 on N-terminus (fewer proteins)

23. PEROXISOME TRANSPORT RECEPTOR
 Peroxins
• Binds to proteins with PTS1 and
docks to the translocation channel
• Complex transported via membrane
• Protein is released
• Peroxin is recycled

25. NUCLEAR TARGETING

26. TRANSPORT INTO NUCLEUS
• All proteins in nucleus are synthesised in cytoplasm
• Examples:
 histones
Ribosomal protein
DNA and RNA polymerase
Transcription factor

27. • Nuclear localization sequence (NLS) is required for transport
• Transport occurs through nuclear pores:
Nuclear import receptor(Importin α and β)
Energy from GTP
GTPase Ran
• fully folded proteins are transported

30. ER TARGETING

31. CO-TRANSLATIONAL TRANSLOCATION
• ER signal sequence (nascent secretory protein) located at the N-
terminus and contains a sequence of hydrophobic amino acids
• Binding to ER is achieved by signal recognition particle(SRP)
• SRP and SRP receptor delivers the nascent/ribosomal polypeptide
complex into translocon (SEC61 complex)
• With GTP binding, SRP is dissociated and translocon further carries
the translation

32. SRP SEC61 complex

34. Post translational translocation (YEAST)

35. INSERTION OF MEMBRANE Pr INTO ER

36. • Membrane and soluble secretory proteins synthesized on the rough
ER undergo four principal modifications before they reach their final
destinations: (1) covalent addition and processing of carbohydrates
(glycosylation) in the ER and Golgi complex,
(2) formation of disulfide bonds in the ER,
(3) proper folding of polypeptide chains and assembly of
multisubunit proteins in the ER, and
(4) specific proteolytic cleavages in the ER, Golgi
complex, and secretory vesicles.

37. # ADDITION AND INITIAL PROCESSING

38. # ACTION OF
PROTEIN
DISULFIDE
ISOMERASE (PDI)

39. #PROTEIN FOLDING

40. # UNFOLDED PROTEIN RESPONSE

41. #MODIFICATION TO MONITER Pr
FOLDING and QUALITY CONTROL

42. THANK YOU