Mass spectrometric analysis of porcine rhodopsin

Melanopsin Carboxy-terminus phosphorylation plasticity and bulk negative charge, not strict site specificity, achieves phototransduction deactivation

Melanopsin is a visual pigment expressed in a small subset of ganglion cells in the mammalian retina known as intrinsically photosensitive retinal ganglion cells (ipRGCs) and is implicated in regulating non-image forming functions such as circadian photoentrainment and pupil constriction and contrast sensitivity in image formation. Mouse melanopsin's Carboxy-terminus (C-terminus) possesses 38 serine and threonine residues, which can potentially serve as phosphorylation sites for a G-protein Receptor Kinase (GRK) and be involved in the deactivation of signal transduction. Previous studies suggest that S388, T389, S391, S392, S394, S395 on the proximal region of the C-terminus of mouse melanopsin are necessary for melanopsin deactivation. We expressed a series of mouse melanopsin C-terminal mutants in HEK293 cells and using calcium imaging, and we found that the necessary cluster of six serine and threonine residues, while being critical, are insufficient for proper melanopsin deactivation. Interestingly, the additional six serine and threonine residues adjacent to the required six sites, in either proximal or distal direction, are capable of restoring wild-type deactivation of melanopsin. These findings suggest an element of plasticity in the molecular basis of melanopsin phosphorylation and deactivation. In addition, C-terminal chimeric mutants and molecular modeling studies support the idea that the initial steps of deactivation and β-arrestin binding are centered around these critical phosphorylation sites (S388-S395). The degree of functional versatility described in this study, along with ipRGC biophysical heterogeneity and the possible use of multiple signal transduction cascades, might contribute to the diverse ipRGC light responses for use in non-image and image forming behaviors, even though all six sub types of ipRGCs express the same melanopsin gene OPN4.

Post-translational modifications and their applications in eye research (Review)

Gene expression is the process by which genetic information is used for the synthesis of a functional gene product, and ultimately regulates cell function. The increase of biological complexity from genome to proteome is vast, and the post-translational modification (PTM) of proteins contribute to this complexity. The study of protein expression and PTMs has attracted attention in the post‑genomic era. Due to the limited capability of conventional biochemical techniques in the past, large‑scale PTM studies were technically challenging. The introduction of effective protein separation methods, specific PTM purification strategies and advanced mass spectrometers has enabled the global profiling of PTMs and the identification of a targeted PTM within the proteome. The present review provides an overview of current proteomic technologies being applied in eye research, with a particular focus on studies of PTMs in ocular tissues and ocular diseases.

GPCR proteomics: mass spectrometric and functional analysis of histamine H1 receptor after baculovirus-driven and in vitro cell free expression

The human histamine H1 Receptor (hH1R) belongs to the family of G-protein coupled receptors (GPCRs), an attractive and proven class of drug targets in a wide range of therapeutic areas. However, due to the low amount of available purified protein and the hydrophobic nature of GPCRs, limited structural information is available on ligand-receptor interaction especially for the transmembrane (TM) domain regions where the majority of ligand-receptor interactions occur. During the last decades, proteomic techniques have increasingly become an important tool to reveal detailed information on the individual GPCR class, including post-translational modifications and characterizations of GPCRs binding pocket. Herein, we report the successful functional production and mass spectrometric characterization of the hH1R, after baculovirus-driven and in vitro cell-free expression. Using only MALDI-ToF, sequence coverage of more than 80%, including five hydrophobic TM domains was achieved. Moreover, we have identified an asparagine residue in the hH1R protein that is subject to N-linked glycosylation. This information would be valuable for drug discovery efforts by allowing us to further study H1R-ligand interactions using histaminergic ligands that covalently bind the hH1R, and eventually revealing binding sites of hH1R and other GPCRs.

MALDI tissue profiling of integral membrane proteins from ocular tissues

MALDI tissue profiling and imaging have become valuable tools for rapid, direct analysis of tissues to investigate spatial distributions of proteins, potentially leading to an enhanced understanding of the molecular basis of disease. Sample preparation methods developed to date for these techniques produce protein expression profiles from predominantly hydrophilic, soluble proteins. The ability to obtain information about the spatial distribution of integral membrane proteins is critical to more fully understand their role in physiological processes, including transport, adhesion, and signaling. In this article, a sample preparation method for direct tissue profiling of integral membrane proteins is presented. Spatially resolved profiles for the abundant lens membrane proteins aquaporin 0 (AQP0) and MP20, and the retinal membrane protein opsin, were obtained using this method. MALDI tissue profiling results were validated by analysis of dissected tissue prepared by traditional membrane protein processing methods. Furthermore, direct tissue profiling of lens membrane proteins revealed age related post-translational modifications, as well as a novel modification that had not been detected using conventional tissue homogenization methods.

Palmitylation of cone opsins

Palmitylation is a widespread modification in G-protein-coupled receptors and often a dynamic process. In rhodopsins, palmitylation is static on C322/C323. Red/green (M/LWS) cone opsins have no cysteines at corresponding positions and no palmitylation. Blue (SWS2) cone opsins have a single corresponding cysteine and mass spectrometric analysis showed partial palmitylation of salamander SWS2 cone opsin. Ultraviolet (SWS1) cone opsins have one corresponding cysteine, but only unpalmitylated opsin was observed for mouse and salamander. The results show that the static palmitylation found on rhodopsin is not found on cone opsins and suggest the possibility of an unidentified role for opsin palmitylation in cones.