14-3-3 proteins: regulation of endoplasmic reticulum localization and surface expression of membrane proteins

Time-course studies of 14-3-3 protein isoforms in cerebrospinal fluid and brain of primates after oral or intracerebral infection with bovine spongiform encephalopathy agent

Experimental transmission of bovine spongiform encephalopathy (BSE) to cynomolgus monkeys (Macaca fascicularis) is an animal model for variant Creutzfeldt–Jakob disease (vCJD). The presence of 14-3-3 proteins in cerebrospinal fluid (CSF) samples indicates neuronal destruction and is therefore used as a clinical biomarker. However, time-course studies using 14-3-3 proteins have not been performed until now in simian vCJD. The main goals of this study were to determine isoform patterns, to examine kinetics and to correlate the clinical course with the occurrence of this biomarker in simian vCJD. In monkeys dosed intracerebrally with BSE, the earliest clinical sign of illness was a drop in body weight that was detected months before the onset of mild neurological signs. Macaques dosed orally or intracerebrally with BSE developed neurological signs 4.3 (3.7–4.6) and 4.8 (2.9–6.0) years post-infection, respectively. 14-3-3β- and -γ-positive CSF samples were found around the time of onset of mild neurological signs, but not earlier. In contrast, 14-3-3ϵ and -ηisoforms were not detectable. 14-3-3 levels increased with time and were positively correlated with the degree of neurological symptoms. Post-mortem examination of brain samples revealed a positive correlation between PrPresand 14-3-3ϵ levels. Interestingly, florid plaques characteristic of human vCJD could not be detected in diseased monkeys. It was concluded that analysis of 14-3-3 proteins in CSF is a reliable tool to characterize the time course of brain degeneration in simian vCJD. However, there are differences in the clinical course between orally and intracerebrally infected animals that may influence the detection of other biomarkers.

Over-expression of 14-3-3zeta is an early event in oral cancer

Background

The functional and clinical significance of 14-3-3 proteins in human cancers remain largely undetermined. Earlier, we have reported differential expression of 14-3-3ζ mRNA in oral squamous cell carcinoma (OSCC) by differential display.

Methods

The clinical relevance of 14-3-3ζ protein in oral tumorigenesis was determined by immunohistochemistry in paraffin embedded sections of oral pre-malignant lesions (OPLs), OSCCs and histologically normal oral tissues and corroborated by Western Blotting. Co-immunoprecipitation assays were carried out to determine its association with NFκB, β-catenin and Bcl-2.

Results

Intense immunostaining of 14-3-3ζ protein was observed in 61/89 (69%) OPLs and 95/120 (79%) OSCCs. Immunohistochemistry showed significant increase in expression of 14-3-3ζ protein from normal mucosa to OPLs to OSCCs (p_trend < 0.001). Significant increase in expression of 14-3-3ζ protein was observed as early as in hyperplasia (p = 0.009), with further elevation in moderate and severe dysplasia, that was sustained in OSCCs. These findings were validated by Western blotting. Using Co-immunoprecipitation, we demonstrated that 14-3-3ζ protein binds to NFκB, β-catenin and Bcl-2, suggesting its involvement in cellular signaling, leading to proliferation of oral cancer cells.

Conclusion

Our findings suggest that over-expression of 14-3-3ζ is an early event in oral tumorigenesis and may have an important role in its development and progression. Thus, 14-3-3ζ may serve as an important molecular target for designing novel therapy for oral cancer.

Protection against beta-amyloid-induced apoptosis by peptides interacting with beta-amyloid

beta-Amyloid peptide produces apoptosis in neurons at micromolar concentrations, but the mechanism by which beta-amyloid exerts its toxic effect is unknown. The normal biological function of beta-amyloid is also unknown. We used phage display, co-precipitation, and mass spectrometry to examine the protein-protein interactions of beta-amyloid in normal rabbit brain in order to identify the biochemical receptors for beta-amyloid. beta-Amyloid was found to bind primarily to proteins involved in low density lipoprotein and cholesterol transport and metabolism, including sortilin, endoplasmic reticulum-Golgi intermediate compartment 2 (ERGIC2), ERGIC-53, steroid 5alpha-reductase, and apolipoprotein B. beta-Amyloid also bound to the C-reactive protein precursor, a protein involved in inflammation, and to 14-3-3, a protein that regulates glycogen synthase kinase-3beta, the kinase involved in tau phosphorylation. Of eight synthetic peptides identified as targets of beta-amyloid, three were found to be effective blockers of the toxic effect of beta-amyloid on cultured neuronal cells. These peptides bound to the hydrophobic region (residues 17-21) or to the nearby protein kinase C pseudo-phosphorylation site (residues 26-30) of beta-amyloid, suggesting that these may be the most critical regions for beta-amyloid effector action and for aggregation. Peptides or other small molecules that bind to this region may protect against beta-amyloid toxic effect by competitively blocking its ability to bind beta-amyloid effector proteins such as sortilin and 14-3-3.

Mechanisms of cardiac potassium channel trafficking

The regulation of ion channels involves more than just modulation of their synthesis and kinetics, as controls on their trafficking and localization are also important. Although the body of knowledge is fairly large, the entire trafficking pathway is not known for any one channel. This review summarizes current knowledge on the trafficking of potassium channels that are expressed in the heart. Our knowledge of channel assembly, trafficking through the Golgi apparatus and on to the surface is covered, as are controls on channel surface retention and endocytosis.

Differential trafficking of carboxyl isoforms of Ca2+-gated (Slo1) potassium channels

The pore-forming subunit of the large-conductance Ca(2+)-dependent K(+) (Slo1) channel is encoded by one gene. However, the functional properties of Slo1 channels are diverse in part because of their numerous regulatory mechanisms including posttranslational modification and alternative splicing. In particular, multiple splice variants of the pore-forming subunit have been reported but their significance is only beginning to be elucidated. Here we examined the cell biological properties of the three common C-terminal isoforms that differ in the last 8 (Slo1_ERL and Slo1_VYR) or 61 residues (Slo1_DEC). We found that Slo1_DEC, the longest isoform, shows dramatically reduced surface expression compared to that of Slo1_ERL or Slo1_VYR. Immunocytochemistry revealed that a large fraction of Slo1_DEC remains localized in endoplasmic reticulum (ER). Using a GST fusion protein containing the Slo1_DEC-specific sequence, affinity purification was carried out to isolate interacting proteins. The identified proteins include protein phosphatase 2A (PP2A-A), actin, and tubulin. The PP2A-A interaction is specific to Slo1_DEC and causes a significant reduction of phosphorylation in Slo1_DEC but not Slo1_ERL or Slo1_VYR. The results together support the notion that Slo1_DEC nucleates isoform-specific protein complexes and possesses a cis element(s) for regulating trafficking of the Slo1 channels.

Scavenging of 14-3-3 proteins reveals their involvement in the cell-surface transport of ATP-sensitive K+ channels

Arginine (Arg)-based endoplasmic reticulum (ER)-localization signals are involved in the quality control of different heteromultimeric membrane protein complexes. ATP-sensitive potassium (KATP) channels are unique because each subunit in the heterooctamer contains an Arg-based ER-localization signal. We have dissected the inactivation events that override the ER-localization activity of the eight peptide-sorting motifs. Employing a 14-3-3-scavenger construct to lower the availability of 14-3-3 proteins, we found that 14-3-3 proteins promote the cell-surface expression of heterologously expressed and native KATP channels. 14-3-3 proteins were detected in physical association with KATP channels in a pancreatic beta-cell line. Our results suggest that the Arg-based signal present in Kir6.2 is sterically masked by the SUR1 subunit. By contrast, 14-3-3 proteins functionally antagonized the Arg-based signal present in SUR1. The last ten amino acids were required for efficient 14-3-3 recruitment to multimeric forms of the Kir6.2 C-terminus. Channels containing a pore-forming subunit lacking these residues reached the cell surface inefficiently but were functionally indistinguishable from channels formed by the full-length subunits. In conclusion, 14-3-3 proteins promote the cell-surface transport of correctly assembled complexes but do not regulate the activity of KATP channels at the cell surface.

14-3-3 protein interacts with Huntingtin-associated protein 1 and regulates its trafficking

HAP1 (Huntingtin-associated protein 1) consists of two alternately spliced isoforms (HAP1A and HAP1B, which have unique C-terminal sequences) and participates in intracellular trafficking. The C terminus of HAP1A is phosphorylated, and this phosphorylation was found to decrease the association of HAP1A with kinesin light chain, a protein involved in anterograde transport in cells. It remains unclear how this phosphorylation functions to regulate the association of HAP1 with trafficking proteins. Using the yeast two-hybrid system, we found that HAP1 also interacts with 14-3-3 proteins, which are involved in the assembly of protein complexes and the regulation of protein trafficking. The interaction of HAP1 with 14-3-3 is confirmed by their immunoprecipitation and colocalization in mouse brain. Moreover, this interaction is specific to HAP1A and is increased by the phosphorylation of the C terminus of HAP1A. We also found that expression of 14-3-3 decreases the association of HAP1A with kinesin light chain. As a result, there is less HAP1A distributed in neurite tips of PC12 cells that overexpress 14-3-3. Also, overexpression of 14-3-3 reduces the effect of HAP1A in promoting neurite outgrowth of PC12 cells. We propose that the phosphorylation-dependent interaction of HAP1A with 14-3-3 regulates HAP1 function by influencing its association with kinesin light chain and trafficking in neuronal processes.

Structural basis for protein-protein interactions in the 14-3-3 protein family

The seven members of the human 14-3-3 protein family regulate a diverse range of cell signaling pathways by formation of protein-protein complexes with signaling proteins that contain phosphorylated Ser/Thr residues within specific sequence motifs. Previously, crystal structures of three 14-3-3 isoforms (zeta, sigma, and tau) have been reported, with structural data for two isoforms deposited in the Protein Data Bank (zeta and sigma). In this study, we provide structural detail for five 14-3-3 isoforms bound to ligands, providing structural coverage for all isoforms of a human protein family. A comparative structural analysis of the seven 14-3-3 proteins revealed specificity determinants for binding of phosphopeptides in a specific orientation, target domain interaction surfaces and flexible adaptation of 14-3-3 proteins through domain movements. Specifically, the structures of the beta isoform in its apo and peptide bound forms showed that its binding site can exhibit structural flexibility to facilitate binding of its protein and peptide partners. In addition, the complex of 14-3-3 beta with the exoenzyme S peptide displayed a secondary structural element in the 14-3-3 peptide binding groove. These results show that the 14-3-3 proteins are adaptable structures in which internal flexibility is likely to facilitate recognition and binding of their interaction partners.