1. 1 CA García Sepúlveda MD PhD Tema 12 Localización y translocación proteica Laboratorio de Genómica Viral y Humana Facultad de Medicina, Universidad Autónoma de San Luis Potosí

2. 2 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 Tema 12. Localización y translocación proteica Introducción

3. 3 Proteins that are localized post-translationally are released into the cytosol after synthesis on free ribosomes. Protein fate Tema 12. Localización y translocación proteica Localización postraduccional

4. 4 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 Tema 12. Localización y translocación proteica Localización postraduccional

5. 5 Proteins localized co-translationally associate with the ER membrane during synthesis, ribosomes are "membrane-bound". Protein fate Tema 12. Localización y translocación proteica Localización co-traduccional

6. 6 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 Tema 12. Localización y translocación proteica Localización co-traduccional

7. 7 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 Tema 12. Localización y translocación proteica Localización co-traduccional

8. 8 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. Tema 12. Localización y translocación proteica Proteinas citosolicas

9. 9 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). Tema 12. Localización y translocación proteica Proteinas citosolicas

10. 10 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 vessicle transport system. –Proteins that are secreted from the cell are transported to and through the plasma membrane to the exterior. Tema 12. Localización y translocación proteica Sistema reticuloendotelial

11. 11 There are three major subdivisions of the endomembrane system – the secretory pathway –the lysosomal pathway and –the endocytotic pathway Tema 12. Localización y translocación proteica Sistema reticuloendotelial

12. 12 There are three major subdivisions of the endomembrane system – the secretory pathway –the lysosomal pathway and –the endocytotic pathway Tema 12. Localización y translocación proteica Sistema reticuloendotelial

13. 13 There are three major subdivisions of the endomembrane system – the secretory pathway –the lysosomal pathway and –the endocytotic pathway Tema 12. Localización y translocación proteica Sistema reticuloendotelial

14. 14 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. Tema 12. Localización y translocación proteica Sistema reticuloendotelial

15. 15 Consists of compartments: –Endoplasmic Reticulum –Golgi apparatus –Lysosomes –Endosomes and –Secretory Vesicles. Tema 12. Localización y translocación proteica Sistema reticuloendotelial

16. 16 Compartments involved in the processing of proteins for: –export from the cell –for lysosomes (destruction) –for proteins entering the cell from the cell surface. Tema 12. Localización y translocación proteica Sistema reticuloendotelial

17. 17 Compartments involved in the processing of proteins for: –export from the cell –for lysosomes (destruction) –for proteins entering the cell from the cell surface. Tema 12. Localización y translocación proteica Sistema reticuloendotelial

18. 18 Compartments involved in the processing of proteins for: –export from the cell –for lysosomes (destruction) –for proteins entering the cell from the cell surface. Tema 12. Localización y translocación proteica Sistema reticuloendotelial

19. 19 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. Tema 12. Localización y translocación proteica Localización proteica

20. 20 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 → Tema 12. Localización y translocación proteica Señales de localización

21. 21 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. Tema 12. Localización y translocación proteica Señales de localización a RER

22. 22 This targeting relies on a signal encoded in the N terminal portion of the protein. Tema 12. Localización y translocación proteica Señales de localización a RER

23. 23 The signal is recognized by a Signal Recognition Particle (SRP). Tema 12. Localización y translocación proteica Señales de localización a RER

24. 24 The SRP enables the ribosome to dock to the corresponding translocator protein (translocon). Tema 12. Localización y translocación proteica Señales de localización a RER

25. 25 The SRP enables the ribosome to dock to the corresponding translocator protein (translocon). Tema 12. Localización y translocación proteica Señales de localización a RER

26. 26 Signal sequence provides the same traffikcing pattern for completely distinct proteins... Tema 12. Localización y translocación proteica Señales de localización a RER

27. 27 Nascent polypeptide is inyected into ER and the signal sequence is cleaved by a Signal Peptidase. Tema 12. Localización y translocación proteica Señales de localización a RER

28. 28 Protein synthesis continues to completion until the ribosome is undocked & dissociated. Tema 12. Localización y translocación proteica Señales de localización a RER

29. 29 This is a prime example of a co-translationally localized protein...now on to explore post-translational localization... Tema 12. Localización y translocación proteica Señales de localización a RER

30. 30 What about proteins synthesized in the cytosol that are incorporated to the ER ? Tema 12. Localización y translocación proteica Señales de localización a RER

31. 31 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. Tema 12. Localización y translocación proteica Señales de localización a RER

32. 32 And what about proteins that become an INTEGRAL PART OF THE ER MEMBRANE ? Tema 12. Localización y translocación proteica Proteinas de membrana

33. 33 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. Tema 12. Localización y translocación proteica Proteinas de membrana

34. 34 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. Tema 12. Localización y translocación proteica Proteinas de membrana

35. 35 Classification based on the way the integral proteins are inserted into the membrane and on the times they pass through it. Tema 12. Localización y translocación proteica Proteinas de membrana

36. 36 Type I : Single pass, N-terminus in extracellular or luminal space. Leader sequence in N-terminus Leader sequence is cleaved inside the ER lumen. Tema 12. Localización y translocación proteica Proteinas de membrana

37. 37 Type II : Single pass, C-terminus in extracellular or luminal space. Leader sequence absent but protein introduced C-terminus first. Tema 12. Localización y translocación proteica Proteinas de membrana

38. 38 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 Tema 12. Localización y translocación proteica Proteinas de membrana

39. 39 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. Tema 12. Localización y translocación proteica Proteinas de membrana

40. 40 Luminal side becomes extracellular side for some proteins. Tema 12. Localización y translocación proteica Proteinas de membrana

41. 41 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. Tema 12. Localización y translocación proteica Proteinas mitocondriales

42. 42 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. Tema 12. Localización y translocación proteica Proteinas mitocondriales

43. 43 The "default pathway" takes a protein through the ER, into the Golgi, and on to the plasma membrane. Tema 12. Localización y translocación proteica Proteinas secretadas

44. 44 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. Tema 12. Localización y translocación proteica Tráfico en células polarizadas

45. 45 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). Tema 12. Localización y translocación proteica Tráfico en células polarizadas

46. 46 Proteins that reside in the ER possess a C-terminal tetrapeptide KDEL (Lys- Asp-Glu-Leu) which signals their return to 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. Tema 12. Localización y translocación proteica Tráfico en células polarizadas

47. 47 –Mitochondria synthesize only ~10 organelle 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. Tema 12. Localización y translocación proteica Jerarquias de secuencias lider

48. 48 –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. Tema 12. Localización y translocación proteica Jerarquias de secuencias lider

49. 49 –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 intermembrane space or as integral inner membrane proteins require even more signals. Tema 12. Localización y translocación proteica Jerarquias de secuencias lider

50. 50 –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 Tema 12. Localización y translocación proteica Jerarquias de secuencias lider

51. 51 –There is a basic problem in passing a (largely) hydrophilic protein through a hydrophobic membrane. –The energetics of the interaction are highly unfavorable. –Translocating proteins move through an aqueous channel (translocon), interacting with the resident (integral) proteins rather than with the lipid bilayer. Tema 12. Localización y translocación proteica Translocones

52. 52 –When the signal sequence enters the translocon, the ribosome attaches, forming a seal so that the pore is not exposed to the cytosol. –Ribosome is bound by the interaction of the Signal Recognition Particle (SRP) and the SRP-receptor. Tema 12. Localización y translocación proteica Translocones

53. 53 –Sec61 Complex is the major component of the translocon: Sec61α Sec61β Sec61γ – Forms cylindrical oligomers (each of 3 to 4 heterotrimers) with a diameter of ~8.5nm and a central pore of ~2 nm. Tema 12. Localización y translocación proteica Translocones

54. 54 –A more complex translocon is required when a protein is inserted into a membrane post- translationally. Sec61 complex Four other Sec proteins Chaperone BiP (a member of the Hsp70 class) Supply of ATP –BiP prevents protein backslash due to Brownian Motion. Tema 12. Localización y translocación proteica Translocones

55. 55 –The nucleus is segregated from the cytoplasm by an envelope consisting of two membranes. –The outer membrane is continuous with the ER in the cytosol. –The two membranes come into contact at openings called nuclear pore complexes (~3000 per cell). Pore provides a water-soluble channel between nucleus and cytoplasm. Nucleus and cytosol have the same ionic milieu ! Tema 12. Localización y translocación proteica Complejo del poro nuclear

56. 56 –Nuclear pores are used for both import and export of material. –Proteins are synthesized in the cytosol so any protein required in the nucleus must be transported there. –Since all RNA is synthesized in the nucleus, the entire cytoplasmic complement of RNA (mRNA, rRNA, tRNA, and other small RNAs) must be exported from the nucleus. Tema 12. Localización y translocación proteica Complejo del poro nuclear

57. 57 –The entire pore complex has a diameter of about 120 nm. –Pore diameter is 50 nm wide and its "depth" is about 200 nm. –Mammalian is 120 MDalton and contains approximately 30 different protein components. Tema 12. Localización y translocación proteica Complejo del poro nuclear

58. 58 –Molecules of <5 kD that are injected into the cytoplasm appear virtually instantaneously in the nucleus. Freely permeable to ions, nucleotides and other small molecules. –Proteins between 5-50 kD diffuse at a rate that is inversely related to their size. Presumably determined by random contacts with the pore. It takes a few hours for the levels of an injected protein to equilibrate between cytoplasm and nucleus. Small proteins can enter the nucleus by passive diffusion (but they may also be actively transported). –Proteins >50 kD in size do not enter the nucleus by passive diffusion. Active transport required for their passage Tema 12. Localización y translocación proteica Complejo del poro nuclear

59. 59 –For a protein to pass through a NPC it must have a special signal sequence. –The most common motif responsible for import into the nucleus is the Nuclear Localization Signal (NLS). –Its presence is necessary and sufficient to sponsor import into the nucleus. –Mutation of the signal can prevent the protein from entering the nucleus –There is no apparent conservation of sequence of NLS signals short, rather basic stretch of amino acids. Proline residue usually breaks α-helix upstream of basic residues. Hydrophobic residues are rare. Tema 12. Localización y translocación proteica Complejo del poro nuclear

60. 60 Tema 12. Localización y translocación proteica Complejo del poro nuclear