Ceramide in Stem Cell Differentiation and Embryo Development: Novel Functions of a Topological Cell-Signaling Lipid and the Concept of Ceramide Compartments
Figure 2
Structure of aPKC and the “flipside” model of ceramide activity. (a) In aPKC (Figure shows PKCζ), the N-terminal (regulatory) and C-terminal (catalytic) moieties are connected by a hinge region. The N-terminus contains a pseudosubstrate (PS) motif and a PB1 domain. The PB1 domain is associated with the polarity protein Par6 that itself binds to Cdc42. The hinge region contains a C1b domain that has been suggested to be associated with ceramide and is a putative binding site for PAR-4. The C-terminal moiety contains the catalytic domain and several phosphorylation sites involved in activation of the enzyme. Most recently, we have constructed a dominant negative mutant from the C-terminus of aPKC (C20ζ-GFP) that binds to ceramide. Therefore, aPKC may contain two distinct ceramide-binding sites. In the inactive state of aPKC, the N-terminal PS motif “folds back” onto the C-terminal catalytic domain and blocks its access to protein substrates. We have proposed that binding to ceramide “opens up” aPKC and primes its activation by phosphorylation or inhibition by PAR-4 (Flipside model of ceramide activity). (b) For the activation reaction, ceramide binding to aPKC is followed by its association with Par6 and Cdc42. PIP2 or 3 may participate in aPKC activation and subcellular translocation by binding to active (GTP-associated) Cdc42 in this complex. This ceramide-aPKC polarity complex (CAP-PC) may inhibit GSK-3β and control cell polarity, process formation/migration, or ciliogenesis as described in Figure 3. In the presence of increased levels of active PAR-4, ceramide-associated aPKC binds to PAR-4 and forms a ceramide-aPKC apoptosis complex (CAP-AC). This complex inhibits aPKC and prevents activation of its downstream targets, NF-κB and Akt. We have proposed that this leads to activation of Bax/Bad and induction of apoptosis.
