Central proteolytic chamber

Translocation of the Polypeptide Substrate to the Central Proteolytic Chamber... 

On the sequence level, HslV shows sequence similarity with the yS-subunits of arch-aebacterial and eukaryotic proteasomes. The crystal structure of E. coli HslV confirmed that individual subunits share the Ntn-hydrolase fold with Thrl at the N-terminus as the nucleophile, just as in proteasomes. Despite these similarities, there are substantial differences between bacterial HslVU and archaebacterial and eukaryotic 20S proteasomes. In contrast to HslVU, 20S proteasomes are assembled from four rings of seven subunits each, that build up a central proteolytic chamber and two flanking antechambers. 

Fig. 2.13 Structure of the 20S proteosome of Thermoplasm acidophilum. The figure shows the schematic structure of the 20S proteosome (Loewe et al., 1995). Four stacked rings can be identified in the 20S proteosome, each consisting of 7 pro-tomers. The two external rings contain 7 copies ofthe 26 kDa a-subunit ofthe proteosome, while the inner rings are composed of 7 copies ofthe 22 kDa -subunit. The rings form a central channel with three chambers. The catalytic centers of proteolytic cleavage are localized on the -subunits ofthe inner chamber and are represented in the figure as spheres.

Fig.I. Structure of the 20Sproteasome. (A) Low-resolution model (1.2 nm) ofthe20S proteasome derived from the crystal structure of the Thermoplasma proteasome (Lowe et al., 1995). The a subunits form the heptameric outer rings the /3 subunits, the inner rings. (B) The same structure cut open along the sevenfold axis to display the two antechambers (AC) and the central chamber (CC) with the 14 active sites (marked in black). The channel openings at the two ends of the cylinder are 1.3 nm in diameter. (C) Schematic representation of the arrangement of subunits within eukaryotic 20S pro-teasomes (Groll etal., 1997). Open boxes represent proteolytically inactive subunits filled boxes, proteolytically active subunits. The single C2 symmetry axis is shown. (D) Fold of a and /3 subunits of the Thermoplasma proteasome (Lowe et al, 1995). A pair of five-stranded /S sheets is flanked on both sides by a-helices. Helices (H) and strands (S) are numbered HO to H5 and SI toSlO. The ft subunits lack helix HO, which occupies the cleft on one side of the /3-sheet sandwich in the a subunits. The active-site threonine (Thr-1) of p subunits is shown in a ball-and-stick representation.

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