The influence of phosphorylation on the activity and structure of the neuronal IQ motif protein, PEP-19

Pcp4l1 contains an auto-inhibitory element that prevents its IQ motif from binding to calmodulin

Purkinje cell protein 4-like 1 (Pcp4l1) is a small neuronal IQ motif protein closely related to the calmodulin-binding protein Pcp4/PEP-19. PEP-19 interacts with calmodulin via its IQ motif to inhibit calmodulin-dependent enzymes and we hypothesized Pcp4l1 would have similar properties. Surprisingly, full-length Pcp4l1 does not interact with calmodulin in yeast two-hybrid or pulldown experiments yet a synthetic peptide constituting only the IQ motif of Pcp4l1 binds calmodulin and inhibits calmodulin-dependent kinase II. A nine-residue glutamic acid-rich sequence in Pcp4l1 confers these unexpected properties. This element lies outside the IQ motif and its deletion or exchange with the homologous region of PEP-19 restores calmodulin binding. Conversion of a single isoleucine (Ile36) within this motif to phenylalanine, the residue present in PEP-19, imparts calmodulin binding onto Pcp4l1. Moreover, only aromatic amino acid substitutions at position 36 in Pcp4l1 allow binding. Thus, despite their sequence similarities PEP-19 and Pcp4l1 have distinct properties with the latter harboring an element that can functionally suppress an IQ motif. We speculate Pcp4l1 may be a latent calmodulin inhibitor regulated by post-translational modification and/or co-factor interactions.

Identification of calcium-independent and calcium-enhanced binding between S100B and the dopamine D2 receptor

S100B is a dimeric EF-hand protein that undergoes a calcium-induced conformational change and exposes a hydrophobic protein-binding surface. Recently S100B was identified as a binding partner of the dopamine D2 receptor in a bacterial two-hybrid screen involving the third intracellular loop (IC3). The low in vivo calcium concentration in bacteria (100–300 nM) suggests this interaction may occur in the absence of calcium. In this work the calcium-sensitive ability for S100B to recruit the IC3 of the dopamine D2 receptor was examined, and regions in both proteins required for complex formation were identified. Peptide array experiments identified the C-terminal 58 residues of the IC3 (IC3-C58) as the major interacting site for S100B. These experiments along with pull-down assays showed the IC3 interacts with S100B in the absence and presence of calcium. ¹H–¹⁵N HSQC experiments were used to identify residues, primarily in helices III and IV, utilized in the IC3-C58 interaction. NMR titration data indicated that although an interaction between apo-S100B and IC3-C58 occurs without calcium, the binding was enhanced more than 100-fold upon calcium binding. Further, it was established that shorter regions within IC3-C58 comprising its N- and C-terminal halves had diminished binding to Ca²⁺-S100B and did not display any observable affinity in the absence of calcium. This indicates that residue or structural components within both regions are required for optimal interaction with Ca²⁺-S100B. This work represents the first example of an S100B target that interacts with both the apo- and calcium-saturated forms of S100B.

Impaired locomotor learning and altered cerebellar synaptic plasticity in pep-19/PCP4-null mice

PEP-19/PCP4 maps within the Down syndrome critical region and encodes a small, predominantly neuronal, IQ motif protein. Pep-19 binds calmodulin and inhibits calmodulin-dependent signaling, which is critical for synaptic function, and therefore alterations in Pep-19 levels may affect synaptic plasticity and behavior. To investigate its possible role, we generated and characterized pep-19/pcp4-null mice. Synaptic plasticity at excitatory synapses of cerebellar Purkinje cells, which express the highest levels of Pep-19, was dramatically altered in pep-19/pcp4-null mice. Instead of long-term depression, pep-19/pcp4-null mice exhibited long-term potentiation at parallel fiber-Purkinje cell synapses. The mutant mice have a marked deficit in their ability to learn a locomotor task, as measured by improved performance upon repeated testing on an accelerating rotarod. Thus, our data indicate that pep-19/pcp4 is a critical determinant of synaptic plasticity in cerebellum and locomotor learning.

Purkinje cell protein 4 positively regulates neurite outgrowth and neurotransmitter release

Purkinje cell protein 4 (PCP4), also called brain-specific polypeptide 19 (PEP19), is a neurospecific, small calmodulin-binding protein that binds both calcium-free and calcium-binding calmodulin to regulate the calmodulin-mediated signal. The expression level of this molecule is decreased in the brain in Alzheimer's disease, Huntington's disease, and alcoholism. However, little is known of the function of PCP4 regarding neuronal or neuroendocrine cell differentiation and neurotransmitter release. To address this, we established a PCP4 tetracycline-inducible rat chromaffin cell line, PC12. When PCP4 expression was induced with doxcycline, neurite outgrowth was significantly advanced in the presence of nerve growth factor (NGF) and dibutyryl cAMP, which was inhibited by W-7, a calmodulin inhibitor, and PD98059, an ERK inhibitor. In addition, size of the cell body also was increased by treatment with NGF in the PCP4-induced PC12 cells. Constitutive and potassium-evoked release of acetylcholine and dopamine was increased and apoptosis induced by hydrogen peroxide (H(2)O(2)) was inhibited in PCP4-induced PC12 cells. On the other hand, knockdown of PCP4 by siRNA transfection decreased neurite outgrowth and dopamine release and increased H(2)O(2)-induced apoptosis in PC12 cells. These results indicate that PCP4 promotes neuroendocrine cell differentiation and neurotransmitter release by activating calmodulin function.

Peptide 19 in the rat superior cervical ganglion

Peptide 19 is a 7.6 kDa polypeptide which can bind to calmodulin and inhibit calcium-calmodulin signaling. In this study, peptide 19-immunoreactivity was examined in the rat superior cervical ganglion. In the ganglion, 54.8% of postganglionic sympathetic neuron profiles were immunoreactive for peptide 19. These neuron profiles were small- to medium-sized and measured 87-845 microm(2) (mean+/-SD = 343+/-111 microm(2)). Double immunofluorescence method revealed that 99.9% of peptide 19-containing neurons had neuropeptide Y in the superior cervical ganglion. Retrograde neuronal tracing and immunohistochemical studies also demonstrated that peptide 19 was common in postganglionic sympathetic neurons which innervated the facial skin and masseter but not the submandibular gland; 55.6% and 75.2% of cutaneous and muscular neuron profiles, respectively, contained peptide 19. Only 9.8% of glandular neurons were immunoreactive for peptide 19. These findings indicate that the content of peptide 19 in superior cervical ganglion neurons depends on their cell sizes and peripheral projections. On the other hand, colchicine injection into the superior cervical ganglion decreased the number of peptide 19-positive neurons (30.7%) compared to saline injection (53.3%). In contrast, the treatment induced nicotine adenine dinucleotide phosphate diaphorase activity in 12.7% of postganglionic sympathetic neurons. Double stain demonstrated that 56.3% of nicotine adenine dinucleotide phosphate diaphorase-positive neurons co-expressed peptide 19. These findings indicate that colchicine treatment causes decrease of peptide 19 expression and increase of nitric oxide synthase activity.

Solution structure of the calponin homology (CH) domain from the smoothelin-like 1 protein: a unique apocalmodulin-binding mode and the possible role of the C-terminal type-2 CH-domain in smooth muscle relaxation

The SMTNL1 protein contains a single type-2 calponin homology (CH) domain at its C terminus that shares sequence identity with the smoothelin family of smooth muscle-specific proteins. In contrast to the smoothelins, SMTNL1 does not associate with F-actin in vitro, and its specific role in smooth muscle remains unclear. In addition, the biological function of the C-terminal CH-domains found in the smoothelin proteins is also poorly understood. In this work, we have therefore determined the solution structure of the CH-domain of mouse SMTNL1 (SMTNL1-CH; residues 346-459). The secondary structure and the overall fold for the C-terminal type-2 CH-domain is very similar to that of other CH-domains. However, two clusters of basic residues form a unique surface structure that is characteristic of SMTNL1-CH. Moreover, the protein has an extended C-terminal alpha-helix, which contains a calmodulin (CaM)-binding IQ-motif, that is also a distinct feature of the smoothelins. We have characterized the binding of apo-CaM to SMTNL1-CH through its IQ-motif by isothermal titration calorimetry and NMR chemical shift perturbation studies. In addition, we have used the HADDOCK protein-protein docking approach to construct a model for the complex of apo-CaM and SMTNL1-CH. The model revealed a close interaction of SMTNL1-CH with the two Ca(2+) binding loop regions of the C-terminal domain of apo-CaM; this mode of apo-CaM binding is distinct from previously reported interactions of apo-CaM with IQ-motifs. Finally, we comment on the putative role of the CH-domain in the biological function of SMTNL1.

IQ motif selectivity in human IQGAP1: binding of myosin essential light chain and S100B

IQGAPs are cytoskeletal scaffolding proteins which link signalling pathways to the reorganisation of actin and microtubules. Human IQGAP1 has four IQ motifs each of which binds to calmodulin. The same region has been implicated in binding to two calmodulin-like proteins, the myosin essential light chain Mlc1sa and the calcium and zinc ion binding protein S100B. Using synthetic peptides corresponding to the four IQ motifs of human IQGAP1, we showed by native gel electrophoresis that only the first IQ motif interacts with Mlc1sa. This IQ motif, and also the fourth, interacts with the budding yeast myosin essential light chain Mlc1p. The first and second IQ motifs interact with S100B in the presence of calcium ions. This clearly establishes that S100B can interact with its targets through IQ motifs in addition to interacting via previously reported sequences. These results are discussed in terms of the function of IQGAP1 and IQ motif recognition.

Expression of Pcp4 gene during osteogenic differentiation of bone marrow mesenchymal stem cells in vitro

In this study, we established an in vitro model of osteogenic-inductive differentiation of rat bone marrow mesenchymal stem cells (BMSCs) to determine the mechanisms and relative gene function underlying BMSCs osteogenesis. Osteoplastic differentiation of the third generation BMSCs was induced with the alpha-minimal essential medium containing beta-glyceraldehyde-3-phosphate, L: -ascorbic acid, dexamethasone and 1,25-2(OH)2 vitamin D3 prior to applying gene chip technology (also called microarray technology) for global gene expression screening. Real-time quantitative PCR (Real-time PCR) was used to determine the temporal profile of mRNA expression of regulated genes during osteogenic differentiation of BMSCs. A bioinformatic analysis was utilized to determine the functional significance of the identified osteogenic-related genes. Purkinje cell protein 4 (Pcp4) mRNA expression was identified by the gene chip screening as being up-regulated during osteoplastic differentiation of BMSCs. Real-time PCR analysis confirmed the increased expression of Pcp4 mRNA expression during osteoplastic differentiation of BMSCs with an upward trend that peaked at day 14. The bioinformatic analysis identified Pcp4 as a gene involved in the deposition of calcium and the modulation of CaM-dependent protein kinase. Thus, we hypothesize that Pcp4 osteoplastic differentiation of BMSCs is mediated in part via Pcp4-induced calcium deposition to form mineral nodules and modulation of certain signal transduction pathways of BMPs.

Neuropeptidomics: expanding proteomics downwards

Biological function is mainly carried out by a dynamic population of proteins and peptides which may be used as markers for disease diagnosis, prognosis and as a guide for effective treatment. The study of proteins is called proteomics and it is generally performed by two-dimensional gel electrophoresis and mass spectrometric methods. However, gel-based proteomics is methodologically restricted from the low mass region, which includes important endogenous peptides. The study of endogenous peptides, peptidomics, is complicated by protein fragments produced post-mortem during conventional sample handling. Nanoflow liquid chromatography and MS, together with improved methods for sample preparation, have been used to semi-quantitatively monitor endogenous peptides in brain tissue. When rapidly heat-denatured brain tissue was analysed, these methods enabled simultaneous detection of hundreds of peptides and the identification of several endogenous peptides not previously described in the literature. In an application of the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) model for Parkinson's disease, the expression of the small protein PEP-19 was compared with controls. The levels were found to be significantly decreased in the striatum of MPTP-treated animals.