Subunits of the 26 S Proteasome

Proteasomes are large complexes with various proteolytic activities. The least complex proteasome of 20 S is found in archebacteria of the genus Thermoplasma and consists of four stacked rings, each a homoheptamer (two outer rings consist of subunit alpha, two inner rings of beta). In the eukaryotic 20 S proteasome, each ring consists of seven different, but closely related subunits.This 20 S cylinder forms the core of the 26 S proteasome, which contains two additional cap structures, which as isolated particles have a sedimentation coefficient of 19 S ("19 S cap complex" or "PA700"). 26 S proteasomes are found both in the cytoplasm and in the nucleus. Purified 20 S proteasomes cleave proteins in an ATP independent manner; 26 S proteasomes need ATP for proteolytic avtivity, their major substrates are polyubiquitinated proteins. While most eubacteria lack a 20S or 26S proteasome, 20S proteasomes consisting of two different subunits each of alpha and beta type) have been purified from Rhodococcus spec. and other Actinomycetales. All eubacteria appear to contain Clp proteases which like the proteasome consist of a proteolytic core structure (two heptameric rings of subunit ClpP) and ATPase subunits (ClpA), which are distant relatives of the AAA superfamily.

By SDS gelelectrophoresis, at least 15 different proteins can be distinguished in the 19 S cap complex which are called S1 to S15, starting with the largest protein (all subunits are conserved between yeast and man except S5b, which is absent in S. cerevisiae). Several components of the cap structures (S4, S6, S7, S8) have been characterized as members of the AAA superfamily belonging to the same family. In the phylogenetic tree, six eukaryotic subfamilies corresponding to different proteasome subunits can clearly be distinguished (S4, S6a, S6b, S7, S8, S10b). Each subfamily contains exactly one S. cerevisiae protein. They all contain an N-terminal coiled coil domain. A seventh subfamily consists of archebacterial proteins (PAN = proteasome activating nucleotidase), indicating that the archebacterial proteasome also has a regulatory cap in vivo. PAN can be isolated in a complex of 650 kDa containing full length and N-terminally shortened PAN forms (beginning at Met 74, within the coiled-coil domain). The complex stimulates proteolysis of the archebacterial and eukaryotic 20S proteasome, it utilizes CTP even better than ATP.

New nomenclature for S. cerevisiae proteasome regulatory subunits

A unified the nomenclature for the components of the 19S cap of the proteasome has been established (Trends Biochem. Sci. 23, 245-246 (1998)). The names for the non-ATPase subunits are RPN1-12 (Regulatory Particle Non-ATPase), those for the ATPases are RPT1-6 (Regulatory Particle Triple-A):

NEW OLD ORF
RPN1 NAS1/HRD2 YHR027C
RPN2 SEN3 YIL075C
RPN3 SUN2 YER021W
RPN4 SON1/UFD5 YDL020C
RPN5 NAS5 YDL147W
RPN6 NAS4 (S9) YDL097C
RPN7   YPR108W
RPN8   YOR261C
RPN9   YDR427W
RPN10 SUN1/MCB1 (S5a) YHR200W
RPN11 MPR1 YFR004W
RPN12 NIN1 YFR052W
RPT1 CIM5/YTA3 (S7) YKL145W
RPT2 YTA5 (S4) YDL007W
RPT3 YTA2/TNT1 (S6b) YDR394W
RPT4 SUG2/PCS1 (S10b) YOR259C
RPT5 YTA1 (S6a) YOR117W
RPT6 SUG1/CIM3 (S8) YGL048C

The RPTs appear to form a ring contacting the 20S core, it seems that every proteasome cap contains all six RPT species. Together with RPN1,2, and 10 (and subunit S5b in humans), they form the "base" of the 19S cap. RPN10 links to the "lid" consisting of the other RPNs. The lid may be responsible for preventing unubiquitinated proteins access to the base. The base alone is able to refold denaturated citrate synthase, its physiological role probably is unfolding substrate proteins and feeding them into the core. The AAA subunits appear to form dimers (S4>S7, S6a>S8, S6b>S10b) via the N-terminus, S4/S7 binds S5b and S2, forming a tetramer in vitro.

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Last edited: August 11, 1999 by KaiFr