2. (2pts each) Consider cargo proteins that are destined for secretion to the ex

Question

2. (2pts each) Consider cargo proteins that are destined for secretion to the extracellular space. Please predict where the cargo will end up (ex. in the cytoplasm, a specific organelle, transport vesicles, multiple places, extracellular space) for: () & (i) Each individual condition; & (i) The combination of both conditions. A. () Normal Sar1 is mutated so it binds tightly to GDP and cannot exchange it for GTP. (This leads to Sar1*-GDP being the only form of the protein in the cell.) (i) COPII coat proteins are mutated so they bind both to active Sart and active ARF1 (ii) Both B. (i) A KDEL signal sequence is added to the C-terminus of the cargo protein. () The signal recognition (ii) Both C. (i) v-SNAREs on COPII coated vesicles are mutated to be unable to bind t-SNAREs (i) A transmembrane domain is added to the cargo protein. (ii) Both particle is mutated so it can't recognize ER signal sequences. D. (i) Receptors in the Golgi are mutated so they are unable to bind to cargo proteins. (i) COPI coat proteins are mutated so they are unable to bind to membranes (i) Both

Explanation / Answer

Answer A (i)

(ii)

(iii)

       If both Sar1 and COPII are mutated, it blocks the transport of protein carrying vesicles from ER          to Cis Golgi. So, protein remains in ER compartment.

C. i) the v-SNAREs on COPII coated vesicles are mutated to be unable to bind t-SNAREs then vesicles with the cargo proteins will bud off from the ER but will never be able to recognize the cognate t-SNARE on the golgi apparatus. Cargo will remain inside the vesicles.

ii) a transmembrane domain is added to the cargo protein, then it will remain embedded in the membrane of of ER. Once the transport vesicles bud off they will remain associated with the membrane till they reach Golgi apparatus. Thereafter it will be transferred once the appropriate trans-SNARE complex forms.

iii) both occurs then the cargo proteins will remain embedded in the membrane of transport vesicles as the vesicles will not fuse with their target membranes.    

Therefore, if

D. i) the receptors in the Golgi are mutated so they are unable to bind to cargo proteins then cargo proteins will be released but not enter into the golgi apparatus. So, they may get diffuse in the cytoplasm.

ii)  COPI coat proteins are mutated so they are unable to bind to membranes then the cargo proteins will remain in the vesicles. COPI proteins mediate a retrograde transport pathway that selectively recycles proteins from the cis-Golgi complex to the ER. So, the vesicles will remain in and around the periphery of intra-golgi network.

iii) both occurs, then cargo proteins will not be transported back to the ER, so it will remain in the cytoplasm.

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