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Another helping hand. In eukaryotes, the 20S proteasome component
in conjunction with the 19S component degrades ubiquitinated
proteins with the hydrolysis of a molecule of ATP. Archaea lack
ubiquitin and the 26S proteasome but do contain a 20S proteasome.
Some archaea also contain an ATPase that is homologous to the
ATPases of the eukaryotic 19S component. This archaeal ATPase
activity was isolated as a 650-kDa complex (called PAN) from the
archaeon Thermoplasma, and experiments were performed to
determine if PAN could enhance the activity of the 20S proteasome
from Thermoplasma as well as other 20S proteasomes.
Protein degradation was measured as a function of time and in the presence of various combinations of components. Graph A shows
the results.

a. What is the effect of PAN on archaeal proteasome activity in the
absence of nucleotides?
b. What is the nucleotide requirement for protein digestion?
c. What evidence suggests that ATP hydrolysis, and not just the
presence of ATP, is required for digestion?
A similar experiment was performed with a small peptide as a
substrate for the proteasome instead of a protein. The results
obtained are shown in graph B.
d. How do the requirements for peptide digestion differ from those of
protein digestion?
e. Suggest some reasons for the difference.
The ability of PAN from the archaeon Thermoplasma to support
protein degradation by the 20S proteasomes from the archaeon
Methanosarcina and rabbit muscle was then examined.
Percentage of digestion of protein substrate (Source of the 20S
proteasome)
Additions Thermoplasma Methanosarcina R
f. Can the Thermoplasma PAN augment protein digestion by the
proteasomes from other organisms?
g. What is the significance of the stimulation of rabbit muscle
proteasome by Thermoplasma PAN?