Several isoforms of protein kinase C (PKC) are degraded by the ubiquitin-proteasome pathway after phorbol ester-mediated activation. However, little is known about the ubiquitin ligase (E3) that targets activated PKCs. We recently showed that an E3 complex composed of HOIL-1L and HOW (LUBAC) generates linear polyubiquitin chains and induces the proteasomal degradation of a model substrate. HOIL-1L has also been characterized as a PKC-binding protein. Here we show that LUBAC preferentially binds activated conventional PKCs and their constitutively active mutants. LUBAC efficiently ubiquitinated activated PKC in vitro, and degradation of activated PKC alpha was delayed in HOIL-1L-deficient cells. Conversely, PKC activation induced cleavage of HOIL-1L and led to down-regulation of the ligase activity of LUBAC. These results indicate that LUBAC is an E3 for activated conventional PKC, and that PKC and LUBAC regulate each other for proper PKC signaling. (c) 2006 Elsevier Inc. All rights reserved.