Subunit topology of the Rhodococcus proteasome

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Abstract
The 20S proteasome, isolated from the nocardioform actinomycete Rhodococcus erythropolis strain NI86/21, is built from two α‐type and two β‐type subunits. In order to probe the subunit topology, we have set up an expression system which allows coexpression of the genes encoding the α‐ and β‐subunits in all possible combinations. The four respective constructs obtained yielded fully assembled and proteolytically active proteasomes. Biochemical, kinetic and electron microscopy analysis allow us to rule out several of the models which were originally envisaged for the subunit topology of the Rhodococcus proteasome. The experiments further indicate that the assembly pathways of the Rhodococcus and of the Thermoplasma proteasome differ in some important details.