Long-lived states to monitor protein unfolding by proton NMR.

The relaxation of long-lived states (LLS) corresponds to the slow return to statistical thermal equilibrium between symmetric and antisymmetric proton spin states. This process is remarkably sensitive to the presence of external spins and can be used to obtain information about partial unfolding of proteins. We detected the appearance of a destabilized conformer of ubiquitin when urea is added to the protein in its native state.

Perturbing the ubiquitin pathway reveals how mitosis is hijacked to denucleate and regulate cell proliferation and differentiation in vivo.

Background: The eye lens presents a unique opportunity to explore roles for specific molecules in cell proliferation, differentiation and development because cells remain in place throughout life and, like red blood cells and keratinocytes, they go through the most extreme differentiation, including removal of nuclei and cessation of protein synthesis. Ubiquitination controls many critical cellular processes, most of which require specific lysines on ubiquitin (Ub). Of the 7 lysines (K) least is known about effects of modification of K6.

Structure of the s5a:k48-linked diubiquitin complex and its interactions with rpn13.

Degradation by the proteasome typically requires substrate ubiquitination. Two ubiquitin receptors exist in the proteasome, S5a/Rpn10 and Rpn13. Whereas Rpn13 has only one ubiquitin-binding surface, S5a binds ubiquitin with two independent ubiquitin-interacting motifs (UIMs).

Avid interactions underlie the Lys63-linked polyubiquitin binding specificities observed for UBA domains.

Ubiquitin (denoted Ub) receptor proteins as a group must contain a diverse set of binding specificities to distinguish the many forms of polyubiquitin (polyUb) signals. Previous studies suggested that the large class of ubiquitin-associated (UBA) domains contains members with intrinsic specificity for Lys63-linked polyUb or Lys48-linked polyUb, thus explaining how UBA-containing proteins can mediate diverse signaling events. Here we show that previously observed Lys63-polyUb selectivity in UBA domains is the result of an artifact in which the dimeric fusion partner, glutathione S-transferase (GST), positions two UBAs for higher affinity, avid interactions with Lys63-polyUb, but not with Lys48-polyUb.

Mutations in the hydrophobic core of ubiquitin differentially affect its recognition by receptor proteins.

Ubiquitin (Ub) is one of the most highly conserved signaling proteins in eukaryotes. In carrying out its myriad functions, Ub conjugated to substrate proteins interacts with dozens of receptor proteins that link the Ub signal to various biological outcomes. Here we report mutations in conserved residues of Ub's hydrophobic core that have surprisingly potent and specific effects on molecular recognition.

Mapping the interactions between Lys48 and Lys63-linked di-ubiquitins and a ubiquitin-interacting motif of S5a.

Numerous cellular processes are regulated by (poly)ubiquitin-mediated signaling events, which involve a covalent modification of the substrate protein by a single ubiquitin or a chain of ubiquitin molecules linked via a specific lysine. Remarkably, the outcome of polyubiquitination is linkage-dependent. For example, Lys48-linked chains are the principal signal for proteasomal degradation, while Lys63-linked chains act as nonproteolytic signals.

Solution conformation of Lys63-linked di-ubiquitin chain provides clues to functional diversity of polyubiquitin signaling.

Diverse cellular events are regulated by post-translational modification of substrate proteins via covalent attachment of one or a chain of ubiquitin molecules. The outcome of (poly)ubiquitination depends upon the specific lysine residues involved in the formation of polyubiquitin chains. Lys48-linked chains act as a universal signal for proteasomal degradation, whereas Lys63-linked chains act as a specific signal in several non-degradative processes.