Proteolysis in the Archaea
Proteolysis is a central cellular function and vital in the ability of cells to adapt to changes in the natural environment. We are interested in mechanisms of energy-dependent proteolysis and how substrate proteins are recognized for degradation in the Archaea (for a review see Maupin-Furlow et al. [medline]). The focus of our current work is an examination of the structure and function of multicatalytic proteasomes from the methanogenic and halophilic archaea (Maupin-Furlow et al. 1995 [medline]; Wilson et al. 1999. [medline]).
Proteasomes are large multicatalytic proteases found in eucarya, archaea, and Gram-positive actinomyces. The 20S proteasome of 600 to 700 kDa is the catalytic component, which is able to hydrolyze peptides and unfolded protein substrates. It is cylindrical in shape and is composed of four stacked-rings of seven subunits each. The rings assemble to form a nanocompartment with three inner chambers. The two outer rings each consist of seven a-type subunits, while the two inner rings each consist of seven b-type subunits. The catalytic activity is confined to the inner chamber, formed by the b-subunits which harbor the active site N-terminal threonine responsible for nucleophilic attack of peptide bonds (Maupin-Furlow et al. 1998. [medline]). The two antechambers may function to maintain protein substrates in an unfolded state during processive hydrolysis.
In eucaryotes, 20S proteasomes assemble with 19S cap complexes to form larger ATP-dependent 26S proteasomes required for degradation of proteins often covalently modified with ubiquitin. The 19S cap complexes appear to be responsible for recognition, unfolding, and translocation of substrate proteins into the catalytic chamber of the 20S proteasome. Although the ubiquitin tagging mechanism appears to be restricted to eucarya, homologues of the ATPase subunits of the 19S cap are highly conserved in the Archaea.
One of these homologues, the archaeal PAN or proteasome-activating nucleotidase, has been shown to assemble into a 550-kDa complex and stimulate the hydrolysis of substrate proteins in the presence of ATP (Wilson et al. 2000. [medline]). We are currently investigating the numerous analogies in proteasome-mediated degradation of proteins between the archaea and eucarya. In addition we are determining how the archaeal proteasome recognizes proteins in the absence of ubiquitin.
University of Florida
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