CA García Sepúlveda MD PhD

1. Protein Localization, Translocation & Trafficking CA García Sepúlveda MD PhD Laboratorio de Genómica Viral y HumanaFacultad de Medicina, Universidad Autónoma de San Luis Potosí

2. Introduction • Proteins can be classified into two general classes with regard to localization: those that are not associated with membranes; and those not-associated with membranes. • Each class can be subdivided further, depending on whether the protein associates with a particular structure in the cytosol or type of membrane. • Proteins can be localized co-translationally or post-translationally. Protein fate

3. Post-translational localization • Proteins that are localized post-translationally are released into the cytosol after synthesis on free ribosomes. Protein fate

4. Post-translational localization • Proteins that are localized post-translationally are released into the cytosol after synthesis on free ribosomes. • Some have signals for targeting to organelles such as the nucleus or mitochondria. Protein fate

5. Co-translational localization • Proteins localized co-translationally associate with the ER membrane during synthesis, ribosomes are "membrane-bound". Protein fate

6. Co-translational localization • Proteins localized co-translationally associate with the ER membrane during synthesis, ribosomes are "membrane-bound". • The proteins pass into the ER along the Golgi and then through the plasma membrane, unless they have signals that cause retention at one of the steps on the pathway. Protein fate

7. Co-translational localization • Proteins localized co-translationally associate with the ER membrane during synthesis, ribosomes are "membrane-bound". • The proteins pass into the ER along the Golgi and then through the plasma membrane, unless they have signals that cause retention at one of the steps on the pathway. • They may also be directed to other organelles, such as endosomes or lysosomes. Protein fate

8. Cytosolic proteins • Cytosolic (or "soluble") proteins carry out functions in the cytosol. • The ribosomes on which these proteins are synthesized are sometimes called "free ribosomes". • The "default" for a protein released from "free" ribosomes is to remain in the cytosol. • To be targeted to a specific location requires an appropriate signal, typically a sequence motif that causes it to be assembled into a macromolecular structure or recognized by a transport system.

9. Cytosolic proteins • Some proteins remain free in the cytosol in quasi-soluble form; others associate with macromolecular cytosolic structures (filaments, microtubules, centrioles, etc). • This class also includes nuclear proteins (which pass into the nucleus through large aqueous pores).

10. Reticuloendothelial system • aka: Endomembrane System • Series of membranous bodies, including ER, Golgi apparatus, endosomes and lysosomes. • Proteins of this system are inserted into the ER and then directed to their particular locations by the vessicletransport system. • Proteins that are secreted from the cell are transported to and through the plasma membrane to the exterior.

11. Reticuloendothelial system • There are three major subdivisions of the endomembrane system • the secretory pathway • the lysosomal pathway and • the endocytotic pathway

12. Reticuloendothelial system • There are three major subdivisions of the endomembrane system • the secretory pathway • the lysosomal pathway and • the endocytotic pathway

13. Reticuloendothelial system • There are three major subdivisions of the endomembrane system • the secretory pathway • the lysosomal pathway and • the endocytotic pathway

14. Reticuloendothelial system • Once proteins enter the endoplasmic reticulum they never return to the cytosol compartment. • They are carried by vesicle transport to the other compartments of the system. • This flow of vesicles is highly regulated.

15. Reticuloendothelial system • Consists of compartments: • Endoplasmic Reticulum • Golgi apparatus • Lysosomes • Endosomes and • Secretory Vesicles.

16. Reticuloendothelial system • Compartments involved in the processing of proteins for: • export from the cell • for lysosomes (destruction) • for proteins entering the cell from the cell surface.

17. Reticuloendothelial system • Compartments involved in the processing of proteins for: • export from the cell • for lysosomes (destruction) • for proteins entering the cell from the cell surface.

18. Reticuloendothelial system • Compartments involved in the processing of proteins for: • export from the cell • for lysosomes (destruction) • for proteins entering the cell from the cell surface.

19. Protein Translocation • The process of inserting into or passing through a membrane is called protein translocation. • Protein translocation is driven by signals intrinsic to the proteins themselves.

20. Translocation Signals • Nuclear localization signals (short sequences within proteins) enable the proteins to pass through nuclear pores. • One type of signal that determines transport to the peroxisome is a very short C-terminal sequence. • Insulin signal peptide →

21. ER Targeting Synthesis of all proteins begins in the cytosol compartment. For proteins entering the secretory or lysosomal pathways, the first step is targeting to the ER.

22. ER Targeting This targeting relies on a signal encoded in the N terminal portion of the protein.

23. ER Targeting The signal is recognized by a Signal Recognition Particle (SRP).

24. ER Targeting The SRP enables the ribosome to dock to the corresponding translocator protein (translocon).

25. ER Targeting The SRP enables the ribosome to dock to the corresponding translocator protein (translocon).

26. ER Targeting Signal sequence provides the same traffikcing pattern for completely distinct proteins...

27. ER Targeting Nascent polypeptide is inyected into ER and the signal sequence is cleaved by a Signal Peptidase.

28. ER Targeting Protein synthesis continues to completion until the ribosome is undocked & dissociated.

29. ER Targeting This is a prime example of a co-translationally localized protein...now on to explore post-translational localization...

30. ER Targeting What about proteins synthesized in the cytosol that are incorporated to the ER ?

31. ER Targeting What about proteins synthesized in the cytosol that are incorporated to the ER ? The peptide moves through the translocon into the lumen of the ER. The signal peptide remains attached to the membrane. Signal peptide is cleaved off by a signal peptidase.Leaving the protein free in the lumen of the ER.

32. ER Targeting And what about proteins that become an INTEGRAL PART OF THE ER MEMBRANE ?

33. ER Targeting As membrane proteins are being translated, they are translocated into the ER until a hydrophobic domain is encountered. Alpha helices serve as a 'stop transfer' signal and leaves the protein inserted in the ER membrane.

34. ER Targeting The orientation of a protein in the membrane is established when it is first inserted into the membrane. This orientation persists all of the way to its final destination. That is, the cytosolic side of membrane remains on the cytosolic side throughout all processes.

35. ER Targeting Classification based on the way the integral proteins are inserted into the membrane and on the times they pass through it.

36. ER Targeting Type I : Single pass, N-terminus in extracellular or luminal space. Leader sequence in N-terminus Leader sequence is cleaved inside the ER lumen.

37. ER Targeting Type II : Single pass, C-terminus in extracellular or luminal space. Leader sequence absent but protein introduced C-terminus first.

38. ER Targeting Type III : Polypeptide crosses the lipid bilayer multiple times (α-helix rich) Even (2,4,6) number of hydrophobic domains N- and C- on same side Odd (1,3,5) number of hydrophobic domains N- and C- on different sides

39. ER Targeting Lipid chain-anchored membrane proteins and GPI-anchored membrane proteins : Associated with the bilayer only by means of one or more covalently attached fatty acid chains.The latter is bound to the membrane by a glycosylphosphatidylinositol (GPI) anchor.

40. ER Targeting Luminal side becomes extracellular side for some proteins.

41. Endosymbiont Targeting • Mitochondrial and chloroplast proteins are synthesized on "free" ribosomes. • They associate with the organelle membranes by means of N-terminal sequences of ~25 amino acids that are recognized by receptors on the organelle envelope. • Because this process takes place after synthesis of the protein has been completed, it is called post-translational translocation.

42. Endosymbiont Targeting • Same as for ER. • Requires specific translocons and SRP. • As endosymbionts have two membranes, two different types of translocons are needed • TOM • TIM • Incorporated proteins can be integrated into membranes as happens for ER proteins.

43. Protein Trafficking • The "default pathway" takes a protein through the ER, into the Golgi, and on to the plasma membrane.

44. Protein Trafficking • A polarized thyroid epithelial cell synthesizing soluble proteins: • Polypeptides generated by RER membrane-bound polysomes, enter the lumen of RER. • Proteins undergo core glycosylation and by interacting with chaperones acquire their conformation. • Proteins are then transported to the Golgi apparatus, where terminal glycosylation and other post-translational reactions take place.

45. Protein Trafficking • In the Trans-Golgi network (TGN), mature proteins undergo sorting processes and are packed into transport vesicles. • The vesicles carry soluble proteins (inside the vesicle) and membrane proteins (as integral vesicle membrane protein).

46. Protein Trafficking • Proteins that reside in the ER possess a C-terminal tetrapeptide KDEL (Lys-Asp-Glu-Leu) which signals their returnto the ER from the Golgi. COPI is a protein that coats vesicles that transports proteins from the cis end of the Golgi complex to the RER. This type of transport is termed retrograde transport.

47. Leader Sequence Hierarchies • Mitochondria synthesize only ~10organelle proteins; chloroplasts ~50. • The majority of organelle proteins are synthesized in the cytosol by free ribosomes. They must then be imported into the organelle. • Post-translational membrane insertion depends on LEADER SEQUENCES. • Leaders for mitochondria/chloroplasts are usually hydrophilic, consisting of uncharged amino acids interrupted by basic amino acids, and lacking acidic amino acids.

48. Leader Sequence Hierarchies • The leader sequence contains all the information needed to localize a protein. • The leader sequence and the transported protein represent domains that fold independently to be recognized by receptors on the organelle envelope. • The attached polypeptide sequence plays no part in recognition of the envelope. • Complexity of endosymbiont proteins = • outer membrane • the intermembrane space • the inner membrane • the matrix.

49. Leader Sequence Hierarchies • A hierarchy of leader signals tells each protein where to localize. • The default endosymbiont pathway for protein localization takes a protein completely into the matrix. • This requires two signals (in the leader): • Organelle recognition & outer membrane passage (first part of the leader sequence). • Inner Membrane recognition & passage (second part). • Proteins that need to be held in intermembranespace or as integral inner membrane proteins require even more signals.

50. Leader Sequence Hierarchies • This requires two signals (in the leader): • Organelle recognition & outer membrane passage (first part of the leader sequence). • Inner Membrane recognition & passage (second part). Many uncharged amino acids Basic amino acids