Archaea proteasomes

As discussed above, the eubacterial HslVU is distantly related in structure to the proteasome found in archaea and eukaryotes. Surprisingly, however, the structural relationship is not reflected in the regulatory properties as will be described in Section 10.3, which focuses on structural studies of the yeast 20S proteasome and its activation, activity, and inhibition. 

S proteasomes are ubiquitous and essential in eukaryotes (Heinemeyer 2000) ubiquitous but not essential in archaea (Ruepp et al. 1998) and rare and non-essential in bacteria (Knipfer and Shrader 1997 Ete Mot et al. 1999). Because of ist relative simplicity the proteasome from the archaeon Thermoplasma acidophilum (Dahlmann et al. 1989) has played a pivotal role in resolving the structure and enzymatic mechanism of 20S proteasomes (see for example Hegerl et al. 1991 Grziwa et al. 1991 Piihler et al. 1992 Zwickl et al. 1992 Jap et al. 1993). 

Another helping hand. In eukaryotes, the 20S proteasome in conjunction with the I9S component degrades ubiquitinated proteins with the hydrolysis of a molecule of ATP. Archaea lack ubiquitin and the 26S proteasome but do contain a 20S proteasome. Some archaea also contain an ATPase that is homologous to the ATPases of the eukaryotic 19S component. This archaeal ATPase activity was isolated as a 650-kd complex (called PAN) from the archaeon Thermoplasma, and experiments were performed to determine if PAN could enhance the activity of the 20S proteasome from Thermoplasma as well as other 20S proteasomes. 

Both the ubiquitin pathway and the proteasome appear to be pres- ent in all eukaryotes. liomologs of the proteasome are found in prokaryotes, although the physiological roles ot these homologs have not been well established. The proteasomes of some archaea are quite similar in overall structure to their eukaryotic counterparts and similarly have 28 subunits (Figure 23.8). In the archaeal proteasome, however, all a outer-ring subunits and all (8 inner-ring subunits are identical in eukaryotes, each ot or p subunit is one of seven different isoforms. This specialization provides dis-... 

PSMB (proteasome subunit fi-type) proteins appear first in the archaea (Haloferax P5uocoeeus) (University of Florida, Gainesville, EL) ", continue to be present in amphioxus and agnatha", and become later in gnathostomata lENy-inducible PSMB proteins (example zebrafish, Danio rerio). ... 

Thermococcus litoralis, Desulfurococcus sp., and T. maritima, all having optimum temperatures of 85°C, have been characterized (22). Proteasomes, intracellular protease heteromultimeric complexes, based on two types of subunits a and writh Mr approaching 1 MDa, are common to the archaea and typically exhibit several different proteolytic specificities (59). Their cellular function relates to the need for protein turnover, particularly under stress conditions. Although proteasomes are fovmd in all eukaryotes, all archaea and some bacteria, their composition can vary. For instance, the hyperthermophile Pyrococcus furiosus has a 20S proteasome formed by one a-protein and two j8-proteins this proteasome has different amounts of each 8-protein depending on assembly temperature (59). 

Proteasomes are multisubunit peptidases found in all eukaryotes and archaea and some bacteria. The 20 S proteasome is found only in actinomycetales. Prokaryotic 20 S proteasome cores are self-compartmentalized peptidases composed of 14 a-subunits and 14 P-subunits, with the N-terminal threonines of the P-subunits providing the protease activity. Core particles from archaea and bacteria are simpler structures with homoheptameric rings of catalytic P-subunits flanked by homoheptameric rings of a-subunits. In bacteria, proteasomes are evolved in protein turnover, but in archaea, their function is unknown. Proteasomes are threonine peptidases (Darwin 2009 Murata et al. 2009). 

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