Mechanisms of ubiquitin-mediated, limited processing of the NF-kappa B1 precursor protein p105

In most cases, target proteins of the ubiquitin system are completely degra ded. In several exceptions, such as the first step in the activation of the transcriptional regulator NF-kappaB, the substrate, the precursor protein p105, is processed in a limited manner to yield the active subunit p50. p50 is derived from the N-terminal domain of p105, whereas the C-terminal doma in is degraded. The mechanisms involved in this unique process have remaine d elusive. We have shown that a Gly-rich region (GRR) at the C-terminal dom ain of p50 is one important processing signal and that it interferes with p rocessing of the ubiquitinated precursor by the 26S proteasome. Also, amino acid residues 441-454 are important for processing under non-stimulated co nditions. Lys 441 and 442 serve as ubiquitination targets, whereas residues 446-454 may serve as a ligase recognition motif. Following I kappaB kinase (IKK)-mediated phosphorylation, the C-terminal domain of p105, residues 91 8-934, recruits the SCFbeta -TrCP ubiquitin ligase, and ubiquitination by t his complex leads to accelerated processing. The two sites appear to be rec ognized under different physiological conditions by two different ligases, targeting two distinct recognition motifs. We have shown that ubiquitin con jugation and processing of a series of precursors of p105 that lack the C-t erminal IKK phosphorylation/TrCP binding domain, is progressively inhibited with increasing number of ankyrin repeats. Inhibition is due to docking of active NF-kappaB subunits to the ankyrin repeat domain in the C-terminal h alf of p105 (I kappaB gamma). Inhibition is alleviated by phosphorylation o f the C-terminal domain that leads to ubiquitin-mediated degradation of the ankyrin repeat domain and release of the anchored subunits. We propose a m odel that may explain the requirement for two sites: a) a basal site that m ay be involved in co-translational processing prior to the synthesis of the ankyrin repeat domain; and b) a signal-induced site that is involved in pr ocessing/degradation of the complete molecule following cell activation, wi th rapid release of stored, transcriptionally active subunits. (C) 2001 Soc iete francaise de biochimie et biologie moleculaire / Editions scientifique s et medicales Elsevier SAS. All rights reserved.