Crystal structure of heat shock locus V (HslV) from Escherichia coli

Abstract

Heat shock locus V (HslV; also called ClpQ) is the proteolytic core of the ATP-dependent protease HslVU in Escherichia coli. It has sequence similarity with the beta-type subunits of the eukaryotic and archaebacterial proteasomes. Unlike these particles, which display 72-point symmetry, it is a dimer of hexamers with 62-point symmetry. The crystal structure of HslV at 3.8-A resolution, determined by isomorphous replacement and symmetry averaging, shows that in spite of the different symmetry of the particle, the fold and the contacts between subunits are conserved. A tripeptide aldehyde inhibitor, acetyl-Leu-Leu-norleucinal, binds to the N-terminal threonine residue of HslV, probably as a hemiacetal, relating HslV also functionally to the proteasomes of archaea and eukaryotes.

Figure 1

Amino acid sequence alignment of the β-subunits of Thermoplasma acidophilum and HslV. Secondary structures are marked as strands (arrows) and helices (cylinders).

Figure 2

Electron density calculated with phases from isomorphous replacement after 3-fold averaging, contoured at 1.0 σ around Thr-1 and overlaid with the final model.

Figure 3

Sphere model of HslV showing the dodecamer of two stacked hexameric rings in a staggered arrangement.

Figure 4

Space-filling representation of HslV drawn with grasp (20). HslV is cut open along the cylinder axis to show the hydrolytic chamber and the bound calpain inhibitor molecules.

Figure 5

Overlay of HslV (red) with the T. acidophilum β-subunit (green) with bound calpain inhibitors. The secondary structural elements are labeled.

Figure 6

Overlay of one hexameric ring of HslV (red) with one heptameric ring of T. acidophilum β-subunits (green).

Figure 7

Side view of a substructure of the stacked hexameric rings of HslV (Upper) and the heptameric rings of T. acidophilum (Lower), respectively. The red subunits are in identical orientations. The topology of the intersubunit contacts is conserved.