Recombinant Human K48-linked Di-ubiquitin

Background: Comprised of two 76 amino acid ubiquitin units joined via an isopeptide bond at K48. X-ray structures reveal a rigid, globular arrangement in the closed state (PDB 3M3J), while an alternative open crystal form shows a solvent-exposed interface (PDB 3NS8). NMR and SAXS confirm pH-dependent interconversion between these states. K48-linked chains of four or more ubiquitin’s are optimal for high-affinity engagement by shuttle factors (e.g., hHR23A) and the 26S proteasome, yet di-ubiquitin already binds the proteasomal receptors Rpn10 and Rpn13 with measurable affinity and is recognized synergistically by the p97–Ufd1–Npl4 segregase complex during ER-associated degradation.