Expression and purification of metabotropic glutamate receptor 7 peptides

Overexpression and purification of a toxic peptide LaIT2 from Japanese scorpion, Liocheles australasiae

In Japan, there are two species of scorpions, Madara scorpion (Isometrus maculatus) and Yaeyama scorpion (Liocheles australasiae), and both of them are living in Yaeyama island. It has been shown that Liocheles australasiae has venom including β-toxin acting on K⁺-channels (β-KTx) (Juichi et al., 2018) [1]. Interestingly, LaIT2, one of the toxins found in the venom of Liocheles australasiae, displays the virulence for insects but almost not for mammals. Until now, molecular mechanism of the functional specificity of LaIT2 is unknown. To clear this issue, we tried to establish the overexpression system of LaIT2 in Rosetta-gami B (DE3) pLysS, which have trxB/gor mutations to induce the disulfide bond formation. In this study, we have succeeded to overexpress the recombinant LaIT2 (rLaIT2) as a thioredoxin (Trx)-tagged protein, and established the purification protocol with Ni²⁺-NTA column chromatography, enterokinase digestion, and HPLC. We succeeded to obtain approximately 0.5 mg of rLaIT2 from the E. coli cells cultured in 1 L of M9 culture medium. Intramolecular disulfide bonding pattern of rLaIT2 was identified by endopeptidase fragmentation and mass spectrometry. rLaIT2 showed insecticidal activity and antimicrobial activity, and these are almost identical to those of natural LaIT2. ¹H-¹⁵N HSQC spectrum of ¹⁵N-labeled rLaIT2 indicated that the rLaIT2 has a stable conformation.

Met872 is the key residue determining the novel binominal binding of metabotropic glutamate receptor 7 to calmodulin

Two mGluR7-derived peptides corresponding to residues 856 to 879 and 856 to 875 are known to bind to Ca²⁺-saturated calmodulin (Ca²⁺/CaM), and their binding manners are thought to differ. Met872 function is believed as one of the anchor residues for CaM-binding only in the shorter peptide. To uncover the role of Met872 in CaM-binding, we prepared a mutant of the long peptide, mGluR7 (M872A), in which Met872 was replaced with Ala. We used the mutant together with the two peptides to perform NMR-titration experiments to monitor interaction with stable isotope-labeled CaM. Interaction of Ca²⁺/CaM with mGluR7 (M872A) caused a spectrum that differed from that of Ca²⁺/CaM with the long peptide, suggesting that Met872 of mGluR7 could be involved in CaM-binding even in the long peptide. Analyses of all NMR data suggested that the binding between Ca²⁺/CaM and mGluR7 occurs in some conformational equilibrium manner. The unique CaM-binding properties caused by Met872 may be related to mGluR7's function.

Easy and efficient protocol for purification of recombinant peptides

Peptide synthesis and purification remains a challenge. Low abundance leads to small yields when peptides are purified from natural sources. On the other hand, synthetic methods are limited by the chemical properties of the amino acids and the concurrent aggregation of peptides. In this paper, we report a versatile, high yielding and general purification method for randomly chosen recombinant peptides of variable sizes (ranging from ∼1.7kDa to ∼10kDa). Expressed as fusion proteins with commonly used tag proteins, these peptides are cleaved by 'PreScission protease' in a volatile buffer that makes concentration and recovery of the peptide easy. Separation of the cleaved peptide is achieved by selective precipitation of the larger tag protein with acetonitrile; leaving the peptide in solution. Our protocol can be used to generate a wide variety of peptides in significant quantities for biochemical, biophysical and physiological studies.

Conformation of the calmodulin-binding domain of metabotropic glutamate receptor subtype 7 and its interaction with calmodulin

Calmodulin (CaM), a Ca(2+)-binding protein, is a well-known regulator of various cellular functions. One of the targets of CaM is metabotropic glutamate receptor 7 (mGluR7), which serves as a low-pass filter for glutamate in the pre-synaptic terminal to regulate neurotransmission. Surface plasmon resonance (SPR), circular dichroism (CD) spectroscopy and nuclear magnetic spectroscopy (NMR) were performed to study the structure of the peptides corresponding to the CaM-binding domain of mGluR7 and their interaction with CaM. Unlike well-known CaM-binding peptides, mGluR7 has a random coil structure even in the presence of trifluoroethanol. Moreover, NMR data suggested that the complex between Ca(2+)/CaM and the mGluR7 peptide has multiple conformations. The mGluR7 peptide has been found to interact with CaM even in the absence of Ca(2+), and the binding is directed toward the C-domain of apo-CaM rather than the N-domain. We propose a possible mechanism for the activation of mGluR7 by CaM. A pre-binding occurs between apo-CaM and mGluR7 in the resting state of cells. Then, the Ca(2+)/CaM-mGluR7 complex is formed once Ca(2+) influx occurs. The weak interaction at lower Ca(2+) concentrations is likely to bind CaM to mGluR7 for the fast complex formation in response to the elevation of Ca(2+) concentration.