K63linkages

K63 linkages refer to a specific type of polyubiquitin chain where ubiquitin molecules are connected through the lysine-63 residue. Unlike the more widely recognized K48-linked chains that tag proteins for proteasomal degradation, K63-linked chains generally function as signaling and scaffolding signals that regulate various cellular processes. The chain length and topology can vary, and many proteins recognize K63 chains through ubiquitin-binding domains to assemble signaling complexes.

Biosynthesis and specificity involve a coordinated set of enzymes. Ubiquitin is activated by an E1 enzyme and transferred to an E2 conjugating enzyme. A specialized E2, the Ubc13–Uev1A (or UEV1B) complex, preferentially builds K63-linked chains in concert with specific E3 ligases. Prominent E3s that catalyze K63 linkages include TRAF6, RNF8, RNF168, and the BRCA1–BARD1 complex, among others. The result is the attachment of K63-linked chains to substrates or to preformed chains, creating platforms for protein–protein interactions.

K63-linked ubiquitination participates in diverse signaling pathways. In the DNA damage response, RNF8 and RNF168 generate K63 chains at double-strand break sites to recruit repair factors such as BRCA1 and 53BP1. In immune and inflammatory signaling, TRAF6- and other E3-mediated K63 chains activate kinases and transcription factors such as NF-κB. K63 chains also regulate endocytosis, vesicle trafficking, and selective autophagy by serving as recruitment signals for adaptor proteins and autophagy receptors.

Deubiquitinating enzymes counteract K63 chains to terminate signaling. CYLD, AMSH, and BRCC36 are notable DUBs that remove K63 linkages, shaping the duration and intensity of pathways driven by K63 ubiquitination.

Dysregulation of K63 signaling has been linked to inflammation, cancer, and DNA repair disorders, making it a focus of ongoing biomedical research.