Deletion analysis of protein kinase Calpha reveals a novel regulatory segment

Increased biological activity of protein Kinase C gamma is not required in Spinocerebellar ataxia 14

Spinocerebellar ataxia (SCA) is an autosomal dominant neurodegenerative disorder characterized by slowly progressive cerebellar dysfunction. Currently, 42 SCA types are known, some of which are caused by CAG repeat expansions, but others are caused by point mutations or deletions. Spinocerebellar ataxia type 14 (SCA14) is caused by missense mutations or deletions in the PRKCG gene, coding for protein kinase C gamma (PKCγ). It is still not well understood how these mutations eventually cause Purkinje cell dysfunction and death. Because PKCγ is a well characterized signaling protein highly expressed in Purkinje cells SCA14 offers the chance to better understand the pathogenesis of Purkinje cell dysfunction and death. Altered biological activity of PKCγ would be the simplest explanation for the disease phenotype. There are indeed indications that the enzymatic activity of mutated PKCγ proteins could be changed. Many mutations found in SCA14 families are located in the regulatory C1B and C1A domain, while a few mutations are also found in the C2 and in the catalytic C3 and C4 domains. For many of these mutations an increased enzymatic activity could be demonstrated in cell-based assays, but it remains unclear whether there is indeed an altered biological activity of the mutated PKCγ proteins within living Purkinje cells. In this study we used the dendritic morphology of developing Purkinje cells to detect increased biological activity of PKCγ after expression of different mutated PKCγ proteins. Our results indicate that two out of three known mutations in the catalytic domain of PKCγ did indeed show increased biological activity. On the other hand, none of the five tested mutations located in the regulatory C1 or the C2 domain showed an increased biological activity. Our findings indicate that SCA14 mutations located in different domains of the PRKCG gene cause SCA14 by different mechanisms and that an increased constitutive activity of PKCγ may be one, but cannot be the only mechanism to cause disease in SCA14.

Dynamic Response of the C2 Domain of Protein Kinase Cα to Ca2+ Binding

Ca²⁺-dependent conserved-region 2 (C2) domains target their host signaling proteins to anionic membranes. The Ca²⁺-binding event is a prerequisite for membrane association. Here, we investigate multiscale metal-ion-dependent dynamics of the C2 domain of protein kinase Cα (C2α) using NMR spectroscopy. Interactions with metal ions attenuate microsecond-timescale motions of the loop regions, indicating that preorganization of the metal-binding loops occurs before membrane insertion. Binding of a full complement of Ca²⁺ ions has a profound effect on the millisecond-timescale dynamics of the N- and C-terminal regions of C2α. We propose that Ca²⁺ binding allosterically destabilizes the terminal regions of C2α and thereby facilitates the conformational rearrangement necessary for full membrane insertion and activation of protein kinase Cα.

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.

New insights into cancer-related proteins provided by the yeast model

Cancer is a devastating disease with a profound impact on society. In recent years, yeast has provided a valuable contribution with respect to uncovering the molecular mechanisms underlying this disease, allowing the identification of new targets and novel therapeutic opportunities. Indeed, several attributes make yeast an ideal model system for the study of human diseases. It combines a high level of conservation between its cellular processes and those of mammalian cells, with advantages such as a short generation time, ease of genetic manipulation and a wealth of experimental tools for genome- and proteome-wide analyses. Additionally, the heterologous expression of disease-causing proteins in yeast has been successfully used to gain an understanding of the functions of these proteins and also to provide clues about the mechanisms of disease progression. Yeast research performed in recent years has demonstrated the tremendous potential of this model system, especially with the validation of findings obtained with yeast in more physiologically relevant models. The present review covers the major aspects of the most recent developments in the yeast research area with respect to cancer. It summarizes our current knowledge on yeast as a cellular model for investigating the molecular mechanisms of action of the major cancer-related proteins that, even without yeast orthologues, still recapitulate in yeast some of the key aspects of this cellular pathology. Moreover, the most recent contributions of yeast genetics and high-throughput screening technologies that aim to identify some of the potential causes underpinning this disorder, as well as discover new therapeutic agents, are discussed.

Protein kinase C intervention: the state of play

Intervention in protein kinase C (PKC) has a chequered history, partly because of the poor selectivity of many inhibitors and partly a reflection of the sometimes antagonistic action of related PKC isoforms. Recent advances in targeting PKC isoforms have come from structural work on isolated kinase domains that have provided opportunities to drive selectivity through structure-based avenues. The promise of isoform selective inhibitors and the rationale for their development are discussed in the broader context of the PKC inhibitor arsenal.

Inhibition of protein kinase C by dequalinium analogues: Structure–activity studies on head group variations

New dequalinium analogues and related heteroaromatic systems were synthesized and evaluated for inhibition of protein kinase Calpha. In vitro assays with recombinant human PKCalpha showed that the number of the aromatic ring head groups as well as their electron-richness, are critical factors that determine potency. The inhibitory strengths of the synthesized compounds are shown to correlate well with Mulliken charges on the head group ring nitrogen atoms making it possible to design likely candidate molecules having improved protein kinase Calpha inhibitory activity.

Scanning Mutagenesis Studies Reveal Multiple Distinct Regions within the Human Protein Kinase C Alpha Regulatory Domain Important for Phorbol Ester-dependent Activation of the Enzyme

While phorbol ester-binding sites within protein kinase C alpha (PKCalpha) have been identified and characterized utilizing fragments of the enzyme, it remains unclear whether additional regions within the enzyme may play an important role in its ability to be activated by phorbol ester. To examine this hypothesis, we generated 20 glutathione-S-transferase-tagged, V1-deficient, human PKCalpha holoenzyme constructs in which tandem six or 12 amino acid residue stretches along the full regulatory domain were changed to alanine residues. Each protein was assessed for its ability to bind phorbol ester and to induce growth repression when its catalytic activity was activated by phorbol ester upon expression in yeast cells. Mutagenesis of residues 99-158 potently reduced phorbol binding, consistent with previously published findings on the importance of the C1b region in phorbol binding. In addition, we identified a number of regions within the PKC regulatory domain that, when mutagenized, blocked the activation of PKC-mediated growth repression by phorbol ester while actually enhancing phorbol ester binding in vitro (residues 33-62, and 75-86). This study thus helps distinguish regions important for phorbol binding from regions important for the ability of phorbol ester to activate the enzyme. Our findings also suggest that multiple regions within C2 are necessary for full activation of the enzyme by phorbol ester, in particular residues 231-254. Finally, three regions, when mutagenized, completely, blocked catalytic domain activity in vivo (residues 33-62, 75-86, and 123-146), underscoring the important role of regulatory domain sequences in influencing catalytic domain function, even in the absence of the V1 region containing the pseudosubstrate sequence. This is the first tandem mutagenesis study for PKC that assesses the importance of regions for both phorbol binding and for phorbol-dependent activation in the context of the entire holoenzyme.

Inhibition of protein kinase C catalytic activity by additional regions within the human protein kinase Calpha-regulatory domain lying outside of the pseudosubstrate sequence

The N-terminal pseudosubstrate site within the protein kinase Calpha (PKCalpha)-regulatory domain has long been regarded as the major determinant for autoinhibition of catalytic domain activity. Previously, we observed that the PKC-inhibitory capacity of the human PKCalpha-regulatory domain was only reduced partially on removal of the pseudosubstrate sequence [Parissenti, Kirwan, Kim, Colantonio and Schimmer (1998) J. Biol. Chem. 273, 8940-8945]. This finding suggested that one or more additional region(s) contributes to the inhibition of catalytic domain activity. To assess this hypothesis, we first examined the PKC-inhibitory capacity of a smaller fragment of the PKCalpha-regulatory domain consisting of the C1a, C1b and V2 regions [GST-Ralpha(39-177): this protein contained the full regulatory domain of human PKCalpha fused to glutathione S-transferase (GST), but lacked amino acids 1-38 (including the pseudosubstrate sequence) and amino acids 178-270 (including the C2 region)]. GST-Ralpha(39-177) significantly inhibited PKC in a phorbol-independent manner and could not bind the peptide substrate used in our assays. These results suggested that a region within C1/V2 directly inhibits catalytic domain activity. Providing further in vivo support for this hypothesis, we found that expression of N-terminally truncated pseudosubstrate-less bovine PKCalpha holoenzymes in yeast was capable of inhibiting cell growth in a phorbol-dependent manner. This suggested that additional autoinhibitory force(s) remained within the truncated holoenzymes that could be relieved by phorbol ester. Using tandem PCR-mediated mutagenesis, we observed that mutation of amino acids 33-86 within GST-Ralpha(39-177) dramatically reduced its PKC-inhibitory capacity when protamine was used as substrate. Mutagenesis of a broad range of sequences within C2 (amino acids 159-242) also significantly reduced PKC-inhibitory capacity. Taken together, these observations support strongly the existence of multiple regions within the PKCalpha-regulatory domain that play a direct role in the inhibition of catalytic domain activity.

Photo-induced inactivation of protein kinase calpha by dequalinium inhibits motility of murine melanoma cells

Dequalinium (DECA) is a potent antitumor agent and inhibitor of protein kinase C (PKC). Previously it was shown that PKCalpha activity in vitro could be irreversibly inhibited when treated with DECA at low micromolar concentrations and irradiated with 366 nm of light. This approach was used to probe the role of intracellular PKC activity in the motility of metastatic murine melanoma B16 F10 cells and as a target for DECA analogs with increasing PKC inhibitory potencies. Pretreatment of a monolayer of B16 F10 cells with 250 nM of a DECA analog in the presence of UV irradiation for 5 min resulted in 1) complete inhibition of cell motility for up to 4 h in a time-lapse motility assay and 40 to 60% inhibition of cell migration in a Boyden chamber, and 2) inhibition by 40 to 60% of intracellular phosphatidylserine/Ca(2+)-dependent PKC catalytic activity, signifying inactivation of a conventional PKC isoform. Because PKCalpha is the only conventional PKC isoform detected in B16 F10 cells, a stably transfected clone expressing a kinase-defective mutant of PKCalpha was developed that exhibited a substantial loss of adhesion and motility and was refractory to further inhibition by DECA. These findings identify PKCalpha catalytic activity both as a mechanistic component of cell motility and adhesion and as a critical intracellular target of DECA. These studies further suggest that the combined use of UV with nanomolar concentrations of DECA offers an effective chemotherapeutic approach to inhibit metastatic behavior of melanoma cells.

Inhibition of protein kinase C(alpha) by dequalinium analogues: dependence on linker length and geometry

Analogues of a bipartite compound, dequalinium (DECA) (quinolinium, 1,1'-(1,10-decanediyl)bis(4-amino-2-methyl diiodide)), were tested for inhibition of protein kinase C(alpha) (PKC(alpha)). In vitro assays of monomeric and dimeric analogues support a model in which DECA inhibits PKC(alpha) by an obligatory two-point contact, a unique mechanism among PKC inhibitors. The presence of unsaturation in the center of the C(10)-alkyl linker produced geometric isomers with different inhibitory potencies: cis IC(50) = 52 +/- 12 microM and trans IC(50) = 12 +/- 3 microM, where the trans isomer was equipotent to that of the saturated C(10)-DECA. DECA analogues with longer, saturated linkers (C(12), C(14), or C(16)) exhibited enhanced inhibitory potencies which reached a plateau with the C(14)-linker (IC(50) = 2.6 +/- 0.2 microM). Metastatic melanoma cells treated with 250 nM C(12)-, C(14)-, or C(16)-DECA and irradiated with long-wave UV light (which causes irreversible inhibition of PKC(alpha) by DECA) confirmed the linker-dependent inhibition of intracellular PKC(alpha) activity.