Scanning Tunneling Microscope Measurement of Proteasome Conductance

The proteasome is an enzyme that sequentially degrades peptides into small fragments, so the ability to make electrical measurements of its conformational fluctuations could lead to an electronic readout of the sequence of single peptide molecules. Here, we report scanning tunneling microscope (STM) measurements of the conductance of the T. acidophilum 20S proteasome core particle (CP). The wild-type CP did not change conductance significantly as a 4 amino acid peptide substrate was added. Larger peptides were digested by a mutant, CP-Δ12, in which 12 residues were deleted from the N terminus of the alpha chains (opening the central pore). The conductance of this molecule decreased significantly in the presence of a denatured pleiotrophin substrate. Control experiments showed that strong bonding of the protein, both to the substrate electrode and the STM probe, was required for conductivity to be observed. It also appears that substantial penetration of the probe into the protein film is required, a problematic constraint on incorporating the CP into a fixed-gap device for technological applications.