Eukaryotes contain a highly conserved multienzyme
system which covalently links a small protein, ubiquitin,
to a variety of intracellular proteins that bear
degradation signals recognized by this system. The
resulting ubiquitin-protein conjugates are degraded
by the 26S proteasome, an ATP-dependent protease.
Pathways that involve ubiquitin play major roles in a
huge variety of processes, including cell differentiation,
cell cycle, and responses to stress. In this article
we briefly review the design of the ubiquitin system,
and describe two recent advances, the finding that
ubiquitin ligases interact with specific components of
the 26S proteasome, and the demonstration that peptides
accelerate their uptake into cells by activating
the N-end rule pathway, one of several proteolytic
pathways of the ubiquitin system.