Isolation and characterization of the prokaryotic proteasome homolog HslVU (ClpQY) from Thermotoga maritima and the crystal structure of HslV

Heat-shock locus VU (Hs1VU) is an ATP-dependent proteolytic system and a prokaryotic homolog of the proteasome. It consists of Hs1V, the protease, and Hs1U, the ATPase and chaperone. We have cloned, sequenced and expressed both protein, components from the hyperthermophile Thermotoga maritima. T maritima Hs1U hydrolyzes a variety of nucleotides in a temperature-dependent manner, with the optimum lying between 75 and 80 degreesC. It is also nucleotide-unspecific for activation of Hs1V against amidolytic and caseinolytic activity. The Escherichia coli and T maritima Hs1U proteins mutually stimulate Hs1V proteins from both sources, suggesting a conserved activation mechanism. The crystal structure of T maritima Hs1V was determined and refined to 2.1-Angstrom resolution. The structure of the dodecameric enzyme is well conserved compared to those from E. coli and Haemophilus influenzae. A comparison of known Hs1V structures confirms the presence of a cation-binding site, although its exact role in the proteolytic mechanism of Hs1V remains unclear. Amongst factors responsible for the thermostability of T maritima Hs1V, extensive ionic interactions/salt-bridge networks, which occur specifically in the T maritima enzyme in comparison to its mesophilic counterparts, seem to play an important role. (C) 2002 Elsevier Science B.V. All rights reserved.