2. Short response (6pts each, 2 sentences MAX) A. The structure of ATP synthase

Question

2. Short response (6pts each, 2 sentences MAX) A. The structure of ATP synthase in mitochondria is remarkably similar to that of the V-ATPase found in lysosomes. How is it that the former generates ATP while the latter hydrolyzes it? B. Please describe two examples of roles that pH plays in organelle function or in protein transport through the endomembrane system. C. Please provide two similarities and two differences in the molecular mechanisms of how proteins are transported into the ER and into the nucleus. D. What phenomenon are the vesicle transport and cisternal maturation models propose explain, and how does this contribute to the proper functioning of the Golgi apparatus?

Explanation / Answer

2 A.

Vacuolar-type H+ -ATPase (V-ATPase)

is a proton pump that uses ATP from ATP hydrolysis to produce a proton gradient. It does not get phosphorylated like ATP synthase which is a proton channel that uses the energy from a proton gradient to produce ATP.

Lysosomal hydrolases are optimized to function at low pH, and this can help protect the cell from these
hydrolases.A low pH inside the vacuole helps in activating enzymes that degrade biological material.

B.

KDEL and M6P receptors have different binding affinities for their cargo at different pH, which helps impose directionality on protein transport and retrieval.

As the physiochemical conditions in the ER does not allow the binding of KDEL substrate with pH gradient (ER- 7.2 and secretory vesicles- 5.7) because of specific V-type H+ ATPase pumps

C.

Possible similarities-
1. Both types of transport use receptor proteins which recognize and bind to proteins based on signal sequences.

2. Both use energy in some form to mediate transport.

3. Both can be performed post-translationally.

Possible differences-
1. ER transport can be co-translational, but nuclear transport must be post.

2. ER transport occurs across the ER membrane, while nuclear transport occurs through nuclear pores.

3. The proteins involved have different identities.

4. The ER is able to accept membrane proteins, but nuclear proteins are soluble.

D.

The cisternal maturation model suggests that vesicles being transported from the ER clump together to form the cis-Golgi network, which eventually transitions into the 3 types of internal cisternae and ultimately dissociates into transport vesicles as the trans-Golgi complex.

While the vesicle transport model suggests that the cisternae remain stationary and materials/proteins are transported between compartments by vesicles.

The true mechanism of Golgi complex formation is the combination of cisternal maturation and vesicle transport model.

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