Initial three-dimensional reconstructions of protein kinase C δ from two-dimensional crystals on lipid monolayers

Intramolecular C2 Domain-Mediated Autoinhibition of Protein Kinase C βII

The signaling output of protein kinase C (PKC) is exquisitely controlled, with its disruption resulting in pathophysiologies. Identifying the structural basis for autoinhibition is central to developing effective therapies for cancer, where PKC activity needs to be enhanced, or neurodegenerative diseases, where PKC activity should be inhibited. Here, we reinterpret a previously reported crystal structure of PKCβII and use docking and functional analysis to propose an alternative structure that is consistent with previous literature on PKC regulation. Mutagenesis of predicted contact residues establishes that the Ca(2+)-sensing C2 domain interacts intramolecularly with the kinase domain and the carboxyl-terminal tail, locking PKC in an inactive conformation. Ca(2+)-dependent bridging of the C2 domain to membranes provides the first step in activating PKC via conformational selection. Although the placement of the C1 domains remains to be determined, elucidation of the structural basis for autoinhibition of PKCβII unveils a unique direction for therapeutically targeting PKC.

Dynamics and Membrane Interactions of Protein Kinase C

Protein kinase C (PKC) is a family of Ser/Thr kinases that regulate a multitude of cellular processes through participation in the phosphoinositide signaling pathway. Significant research efforts have been directed at understanding the structure, function, and regulatory modes of the enzyme since its discovery and identification as the first receptor for tumor-promoting phorbol esters. The activation of PKC involves a transition from the cytosolic autoinhibited latent form to the membrane-associated active form. The membrane recruitment step is accompanied by the conformational rearrangement of the enzyme, which relieves autoinhibitory interactions and thereby allows PKC to phosphorylate its targets. The multidomain structure and intrinsic flexibility of PKC present remarkable challenges and opportunities for the biophysical and structural biology studies of this class of enzymes and their interactions with membranes, the major focus of this Current Topic. I will highlight the recent advances in the field, outline the current challenges, and identify areas where biophysics and structural biology approaches can provide insight into the isoenzyme-specific regulation of PKC activity.

The C-terminal V5 domain of Protein Kinase Cα is intrinsically disordered, with propensity to associate with a membrane mimetic

The C-terminal V5 domain is one of the most variable domains in Protein Kinase C isoforms (PKCs). V5 confers isoform specificity on its parent enzyme through interactions with isoform-specific adaptor proteins and possibly through specific intra-molecular interactions with other PKC domains. The structural information about V5 domains in solution is sparse. The objective of this work was to determine the conformational preferences of the V5 domain from the α isoform of PKC (V5α) and evaluate its ability to associate with membrane mimetics. We show that V5α and its phosphorylation-mimicking variant, dmV5α, are intrinsically disordered protein domains. Phosphorylation-mimicking mutations do not alter the overall conformation of the polypeptide backbone, as evidenced by the local nature of chemical shift perturbations and the secondary structure propensity scores. However, the population of the “cis-trans” conformer of the Thr⁶³⁸-Pro⁶³⁹-Pro⁶⁴⁰ turn motif, which has been implicated in the down-regulation of PKCα via peptidyl-prolyl isomerase Pin1, increases in dmV5α, along with the conformational flexibility of the region between the turn and hydrophobic motifs. Both wild type and dmV5α associate with micelles made of a zwitterionic detergent, n-dodecylphosphocholine. Upon micelle binding, V5α acquires a higher propensity to form helical structures at the conserved “NFD” motif and the entire C-terminal third of the domain. The ability of V5α to partition into the hydrophobic micellar environment suggests that it may serve as a membrane anchor during the PKC maturation process.

Regulated binding of importin-α to protein kinase Cδ in response to apoptotic signals facilitates nuclear import

PKCδ translocates into the nucleus in response to apoptotic agents and functions as a potent cell death signal. Cytoplasmic retention of PKCδ and its transport into the nucleus are essential for cell homeostasis, but how these processes are regulated is poorly understood. We show that PKCδ resides in the cytoplasm in a conformation that precludes binding of importin-α. A structural model of PKCδ in the inactive state suggests that the nuclear localization sequence (NLS) is prevented from binding to importin-α through intramolecular contacts between the C2 and catalytic domains. We have previously shown that PKCδ is phosphorylated on specific tyrosine residues in response to apoptotic agents. Here, we show that phosphorylation of PKCδ at Tyr-64 and Tyr-155 results in a conformational change that allows exposure of the NLS and binding of importin-α. In addition, Hsp90 binds to PKCδ with similar kinetics as importin-α and is required for the interaction of importin-α with the NLS. Finally, we elucidate a role for a conserved PPxxP motif, which overlaps the NLS, in nuclear exclusion of PKCδ. Mutagenesis of the conserved prolines to alanines enhanced importin-α binding to PKCδ and induced its nuclear import in resting cells. Thus, the PPxxP motif is important for maintaining a conformation that facilitates cytosplasmic retention of PKCδ. Taken together, this study establishes a novel mechanism that retains PKCδ in the cytoplasm of resting cells and regulates its nuclear import in response to apoptotic stimuli.

Site-specific regulation of CA(V)2.2 channels by protein kinase C isozymes betaII and epsilon

Ca(v)2.2 high voltage-gated calcium channels are regulated by phorbol-12-myristae, 13-acetate (PMA) via Ser/Thr protein kinase C (PKC) phosphorylation sites in the I-II linker and C-terminus of the alpha(1) 2.2 subunit. Here we show that PMA enhancement of Ca(v)2.2 currents expressed in Xenopus oocytes can be blocked by inhibitors of PKC betaII or PKC epsilon isozymes, as shown previously for Ca(v)2.3 currents, and that microinjection of PKC betaII or PKC epsilon isozymes in the oocytes expressing the WT Ca(v)2.2 channels increases the basal barium current (I(Ba)). The I-V plot shows a large increase in current amplitude with PKC betaII and PKC epsilon isozymes with only a small shift in the peak I(Ba) in the hyperpolarizing direction. The potentiation of Ca(v)2.2 currents by microinjection of PKC betaII and PKC epsilon isozymes was not altered by the inhibition of G proteins with GDPbetaS. The combination of isozyme specific inhibitors with previously generated Ser/Thr to Ala mutants of alpha(1) 2.2 subunit revealed that PKC betaII or PKC epsilon isozymes (but not PKC alpha or delta) can provide full enhancement through the stimulatory site (Thr-422) in the I-II linker but that PKC epsilon is better at decreasing channel activity through the inhibitory site Ser-425. The enhancing effect of PKC betaII or epsilon at Thr-422 is dominant over the inhibitory effect at Ser-425. Injected PKC betaII also enhances Ca(v)2.2 current when any of the potential stimulatory sites (Ser-1757, Ser-2108 and Ser-2132) are available in the C-terminus. PKC epsilon provides lesser enhancement with C-terminal sites and only with Ser-2108 and Ser-2132. Sites Ser-1757 and Ser-2132, but not Ser-2108, are dominant over the inhibitory site Ser-425. Collectively, these results reveal a hierarchy of regulatory sites in Ca(v)2.2 channels. Site-specific regulation by different PKC isozymes may allow graded levels of channel activation and susceptibility or resistance to subsequent stimulatory events.

Regulation of ghrelin structure and membrane binding by phosphorylation

The peptide hormone ghrelin requires Ser-3 acylation for receptor binding, orexigenic and anti-inflammatory effects. Functions of desacylghrelin are less well understood. In vitro kinase assays reveal that the evolutionarily conserved Ser-18 in the basic C-terminus is an excellent substrate for protein kinase C. Circular dichroism reveals that desacylghrelin is approximately 12% helical in aqueous solution and approximately 50% helical in trifluoroethanol. Ser-18-phosphorylation, Ser-18-Ala substitution, or Ser-3-acylation reduces the helical character in trifluoroethanol to approximately 24%. Both ghrelin and desacylghrelin bind to phosphatidylcholine:phosphatidylserine sucrose-loaded vesicles in a phosphatidylserine-dependent manner. Phosphoghrelin and phosphodesacylghrelin show greatly diminished phosphatidylserine-dependent binding. These results are consistent with binding of ghrelin and desacylghrelin to acidic lipids via the basic face of an amphipathic helix with Ser-18 phosphorylation disrupting both helical character and membrane binding.