Proteasome Complexes

Controlled protein degradation has emerged as a key mechanism to control a vast variety of vital cellular functions. In parallel with signal transduction pathways and the control of gene transcription, the timed degradation of a given protein is a crucial mechanism both to induce and terminate cellular responses. Eukaryotic cells contain two major proteolytic systems, the lysosomal proteases and the 26S proteasome. While the lysosome represents a specialized organelle enclosing many monomeric proteases, the proteasome is a highly complex oligomeric structure that freely diffuses between the cytosol and the nucleus of eukaryotic cells.

The ATP-dependent 2.5 MDa proteasome is composed of two major building blocks, the core particle (CP), which contains the actual proteolytic sites and the regulatory particle (RP), which is required for substrate recognition. The crystal structure of the CP from S. cerevisiae has been solved. Its overall architecture is a barrel-shaped cylinder formed by four stacked heptameric rings. The two outer rings are formed by seven different α-subunits, the two inner rings by seven different β-subunits, three of them harboring the proteolytic active sites. Each of the three active β-subunits cleaves different peptide bonds, resulting in a very broad substrate specificity. To prevent indiscriminate protein degradation, the active sites are buried within the central cavity of the CP cylinder and the entrance gate into the CP cylinder is sealed by N-terminal extensions of the α-subunits. The regulatory particle consists of two groups of subunits, six conserved ATPases and 12 non-related subunits of largely unknown function. In analogy to bacterial orthologues of the proteasomal ATPases, they are thought to form a ring structure, which physically interacts with the core particle. They furthermore posses a chaperone-like activity, which is required to unfold a bound substrate, making it suitable for passage through the narrow pore of the CP. The non-ATPase subunits of the RP serve as recognition sites for poly-ubiquitinated proteins, the major class of substrates for the proteasome.

A crucial control mechanism for proteasomal degradation is the regulated activation of the CP for degradation of substrates. Through physical interaction of activating complexes with the CP, the gate into the central proteolytic cavity is opened allowing substrates to be degraded. In mammalian cells four different proteasome activators have been identified, PA700/RP, PA28α/β, PA28γ and PA200. The crystal structure of a heterologous complex formed by Trypanosoma brucei PA26 (homologous to mammalian PA28α/β) and CP isolated from S. cerevisiae demonstrates the mechanism of activation. Loop extensions from the PA26 subunits dock into surface crevices of the CP α-subunits thereby inducing a conformational change within their N-terminal extensions. These peptides adopt an ordered conformation and open a pore into the CP cylinder. This mechanism most likely is the basis for the activating properties of all known activating complexes. The ability of the CP to interact with various activating complexes adds an additional layer of proteasome regulation, which in mammalian cells might be tissue specific or dependent on distinct stimuli.

PA28

Ecm29