| Abstract: | The 26 S proteasome is a large proteolytic machine, which degrades most
intracellular proteins. We found that thioredoxin, Txnl1/TRP32, binds to
Rpn11, a subunit of the regulatory complex of the human 26 S proteasome. Txnl1
is abundant, metabolically stable, and widely expressed and is present in the
cytoplasm and nucleus. Txnl1 has thioredoxin activity with a redox potential
of about-250 mV. Mutant Txnl1 with one active site cysteine replaced by serine
formed disulfide bonds to eEF1A1, a substrate-recruiting factor of the 26 S
proteasome. eEF1A1 is therefore a likely physiological substrate. In response
to knockdown of Txnl1, ubiquitin-protein conjugates were moderately
stabilized. Hence, Txnl1 is the first example of a direct connection between
protein reduction and proteolysis, two major intracellular protein quality
control mechanisms.Degradation of proteins in eukaryotic cells plays a pivotal role in the
regulation of several important processes, including cell division, antigen
presentation, and signal transduction
(1). Most intracellular
proteins are degraded by the 26 S proteasome, a 2.5-MDa protease complex
composed of more than 30 different subunits
(2).To become degraded, proteins are typically first conjugated to a chain of
ubiquitin moieties. This reaction is catalyzed by ubiquitin ligases. The
ubiquitin chains lend the proteins affinity for the 26 S proteasome
(3). For efficient degradation,
certain ubiquitylated proteins are shuttled to the 26 S proteasome by
substrate recruiting factors, such as Rad23, Dsk2, and eEF1A
(4,
5).The 26 S proteasome is composed of two stable subcomplexes, the
proteolytically active 20 S core and 19 S regulatory complexes, which bind to
one or both ends of the cylindrical 20 S core particle
(6). The regulatory complexes
first recognize the ubiquitylated substrates
(3), before the substrates are
deubiquitylated (7,
8), unfolded
(9,
10), and translocated into the
20 S particle for degradation.Although the 26 S proteasome has been known for more than 20 years
(11), novel subunits and
cofactors have been described recently
(12,
13). Here we report another
novel proteasome-associated protein, Txnl1 (thioredoxin-like protein 1), that
associates directly with the proteasome subunit Rpn11. Txnl1 exhibits
thioredoxin activity and targets eEF1A1 in vivo. Previous reports
have shown that eEF1A1 transfers misfolded nascent proteins from the ribosome
to the 26 S proteasome for degradation
(5,
14,
15). Accordingly,
ubiquitin-protein conjugates were stabilized upon knockdown of Txnl1
expression. Txnl1 therefore directly links protein reduction and proteolysis,
two major intracellular protein quality control mechanisms. |
