How protein is transported in body although it formed only in cytoplasm of cell and explained with the example of transport of nascent protein into Endoplasmic Reticulum through SRP receptor and signal sequence. And what happens when this machinery not works what happen and Examples of diseases because of it.

1. PROTEIN
TOPOGENESIS
Pradhyum Khandelwal
16BBT0118
2/25/2018
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2. SOME QUESTION YOU CAN ASK ABOUT?
Post translation transport Co translational transport i.e.
• From the birth place to the destination how protein travels?
• How from one place to make protein but many destinations to reach?
• How does a protein know where to go and how to reach their “work place”?
Answer is Topogensis
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3. WHAT IS PROTEIN TOPOGENESIS
• morphogenesis of protein tertiary structure
• Eukaryotic cells contain many subcellular structures that serve to carry out
various processes. It requires that each of these compartments is provided with
specific sets of proteins necessary to perform such tasks. With minor exceptions
concerning certain organelles, all proteins are synthesized in the cytoplasm from
where they are distributed to their various locations (see Fig. 1). This process is
referred to as protein topogenesis.
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4. WHY IT IS INPORTANT
• An important aspect of biosynthesis as well as biodegradation is the intracellular
topology of proteins. Many protein species spend their entire life in the same
compartment in which they are synthesized, others have to be translocated across
the hydrophobic barrier of one or two distinct cellular membranes in order to
reach the intracellular compartment or extracellular site where they exert their
function.
• The collective term "topogenesis" has been introduced to encompass protein
translocation (partial or complete) across membranes as well as subsequent post
translocational protein traffic.
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5. CONTINUE…
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6. CONDITION OF COTRANSLTIONAL TRANSPORT
• SRP - signal recognition particle - Particle made of 1 RNA and six proteins
which binds the signal sequence on newly made proteins
• SRP receptor - ER protein which binds SRP
• TRAM - translocating chain-associated membrane (also translocon) takes the
protein across the ER membrane.
• Channels formed by several sub units of proteins which span the membrane.
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7. SIGNAL SEQUENCE
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• A typical signal sequence attached to the N-terminal end of a protein destined to
be transported through the ER membrane. Shows same general pattern of AA- no
specific AA sequence.
• Acidic AA are not present.
• This is recognised by the signal recogniton partical, an RNA protein complex in
the cytoplasm that binds to the signal peptide when it emerges from the
ribosomal tunnel

8. CONTINUE….
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9. STRUCTURE OF THE SIGNAL RECOGNITION PARTICLE
(SRP)
• SRP is a ribonucleoprotein haing300 base RNA molecule and 6 proteins.
• Methionine -on P54 subunit bind to the hydrophobic signal sequence on the
emerging polypeptide
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10. CONTINUE…..
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11. MECHANISM
Step-1
Translation of mRNA begins on free ribosomes for an ER targeted
protein
• N-terminal signal sequence emerges from ribosome tunnel
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12. CONTINUE…..
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13. CONTINUE….
Step-2
• Signal recognition particle (SRP) binds to the emerging signal sequence from the
ribosome and arrest further elongation of polypeptide chain
• Receptor is an α and β dimer – β subunit is an intrinsic membrane protein
• α-subunit initiates binding of ribosome-SRP to ER membrane
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14. CONTINUE…..
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15. CONTINUE….
Step-3
• The loaded ribosome binds to ER membrane which has on it SRP receptor to
which GDP is bound.
• This complex position the ribosome on the membrane.
• Signal sequence binds to translocon. Translocon gate opens and contact with
signal peptide. This contact will trigger the next step.
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16. CONTINUE…..
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17. CONTINUE….
Step-4
• The exchange of GDP- GTP catalysed by an exchange enzyme present in the
cytoplasm.
• In this form the receptor bind more tightly to the SRP and allow the SRP to
detach from the signal peptide – the result of conformational change due to
energy release.
• The signal peptide is is now free to insert into the translocon and SRP is further
released in the cytosol for further use.
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18. CONTINUE…..
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19. CONTINUE….
Step-5
• The ribosome recommenses synthesis of Polypeptide chain which traverse via
translocan channel as it is synthesized.
• The clevage site is exposed at the internal face of the membrane. The signal
peptidase responsible for the clevage has a hydrophobic patch which attaches it
on the membrane so that the signal peptide cleavage site emerges from the
membrane and get detached.
• Signal peptide get degraded in ER lumen.
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20. CONTINUE…..
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21. CONTINUE….
Step-6
• Peptide chain elongation extrudes protein into ER lumen.
• Ribosome dissociates and is released from membrane when protein is completed
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22. CONTINUE…..
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23. CONTINUE….
Step -7 & 8
• Ribosome get ditached from translocon ans start synthesizing new protein.
• Protein get folded in ER lumen and transported through vesicals.
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24. WHOLE PROCEDURE
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25. WHAT CONTROLS THE INSERTION OF NASCENT
SECRETORY PROTEINS INTO THE TRANSLOCON?
• The P54 subunit of the Signal Recognition Particle is a GTPase.
• So is the a-subunit of the SRP receptor.
• GTP hydrolysis initiates protein transport into the ER
GTP binding to both proteins produces conformational changes
required for tight “docking” to the membrane
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26. GTP HYDROLYSIS POWERS
1) Dissociation of SRP, SRP receptor from translocon.
2) Opening of translocon gate.
3) Transfer of signal sequence to translocon.
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27. DIFFERENT TOPOGENSIS
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28. EXAMPLE OF WHAT HAPPENS WHEN PROTEIN
TARGETING DOESN’T WORK?
I-cell disease
Caused by defect in lysosomal targeting.
• Deficiency in tagging of lysosomal enzymes
• Many hydrolytic enzymes fail to be targeted to lysosomes and are secreted from
cells
• Psychomotor retardation, skeletal abnormalities
• Average lifespan ~ 8 years.
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29. CONTINUE…..
Zellwenger syndrome
Caused by defect in Peroxisomal targeting.
• Peroxisomal enzymes accumulate in cytosol
• Neural, cardiovascular, renal, adrenal dystrophies
• Accumulate very long chain fatty acids
• Cataracts, glaucoma, retinal detachment
• Average lifespan - 12 weeks
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Thank you