Mutation and analysis of prenylation signal sequences

GPCR regulation of secretion

Modulation of neurotransmitter exocytosis by activated Gi/o coupled G-protein coupled receptors (GPCRs) is a universal regulatory mechanism used both to avoid overstimulation and to influence circuitry. One of the known modulation mechanisms is the interaction between Gβγ and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNAREs). There are 5 Gβ and 12 Gγ subunits, but specific Gβγs activated by a given GPCR and the specificity to effectors, such as SNARE, in vivo are not known. Although less studied, Gβγ binding to the exocytic fusion machinery (i.e. SNARE) provides a more direct regulatory mechanism for neurotransmitter release. Here, we review some recent insights in the architecture of the synaptic terminal, modulation of synaptic transmission, and implications of G protein modulation of synaptic transmission in diseases. Numerous presynaptic proteins are involved in the architecture of synaptic terminals, particularly the active zone, and their importance in the regulation of exocytosis is still not completely understood. Further understanding of the Gβγ-SNARE interaction and the architecture and mechanisms of exocytosis may lead to the discovery of novel therapeutic targets to help patients with various disorders such as hypertension, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and acute/chronic pain.

Quantitative Multiple-Reaction Monitoring Proteomic Analysis of Gβ and Gγ Subunits in C57Bl6/J Brain Synaptosomes

Gβγ dimers are one of the essential signaling units of activated G protein-coupled receptors (GPCRs). There are five Gβ and 12 Gγ subunits in humans; numerous studies have demonstrated that different Gβ and Gγ subunits selectively interact to form unique Gβγ dimers, which in turn may target specific receptors and effectors. Perturbation of Gβγ signaling can lead to impaired physiological responses. Moreover, previous targeted multiple-reaction monitoring (MRM) studies of Gβ and Gγ subunits have shown distinct regional and subcellular localization patterns in four brain regions. Nevertheless, no studies have quantified or compared their individual protein levels. In this study, we have developed a quantitative MRM method not only to quantify but also to compare the protein abundance of neuronal Gβ and Gγ subunits. In whole and fractionated crude synaptosomes, we were able to identify the most abundant neuronal Gβ and Gγ subunits and their subcellular localizations. For example, Gβ₁ was mostly localized at the membrane while Gβ₂ was evenly distributed throughout synaptosomal fractions. The protein expression levels and subcellular localizations of Gβ and Gγ subunits may affect the Gβγ dimerization and Gβγ-effector interactions. This study offers not only a new tool for quantifying and comparing Gβ and Gγ subunits but also new insights into the in vivo distribution of Gβ and Gγ subunits, and Gβγ dimer assembly in normal brain function.

A minimalist substrate for enzymatic peptide and protein conjugation

Recently a number of nonnatural prenyl groups containing alkynes and azides have been developed as handles to perform click chemistry on proteins and peptides ending in the sequence "CAAX", where C is a cysteine that becomes alkylated, A is an aliphatic amino acid and X is any amino acid. When such molecules are modified, a tag containing a prenyl analogue and the "CAAX box" sequence remains. Here we report the synthesis of an alkyne-containing substrate comprised of only nine nonhydrogen atoms. This substrate was synthesized in six steps from 3-methylbut-2-en-1-ol and has been enzymatically incorporated into both proteins and peptides by using protein farnesyltransferase. After prenylation the final three amino acids required for enzymatic recognition can be removed by using carboxypeptidase Y, leaving a single residue (the cysteine from the "CAAX box") and the prenyl analogue as the only modifications. We also demonstrate that this small tag minimizes the impact of the modification on the solubility of the targeted protein. Hence, this new approach should be useful for applications in which the presence of a large tag hinders the modified protein's solubility, reactivity, or utility.

Proteomic analysis of bovine brain G protein gamma subunit processing heterogeneity

We characterized the variable processing of the G protein gamma subunit isoforms associated with bovine brain G proteins, a primary mediator of cellular communication. Ggamma subunits were isolated from purified brain G proteins and characterized by Edman sequencing, by MALDI MS, by chemical and/or enzymatic fragmentation assayed by MALDI MS, and by MS/MS fragmentation and sequencing. Multiple forms of six different Ggamma isoforms were detected. Significant variation in processing was found at both the amino termini and particularly the carboxyl termini of the proteins. All Ggamma isoforms contain a carboxyl-terminal CAAX motif for prenylation, carboxyl-terminal proteolysis, and carboxymethylation. Characterization of these proteins indicates significant variability in the normal processing of all of these steps in the prenylation reaction, including a new variation of prenyl processing resulting from cysteinylation of the carboxyl terminus. These results have multiple implications for intracellular signaling mechanisms by G proteins, for the role of prenyl processing variation in cell signaling, and for the site of action and consequences of drugs that target the prenylation modification.

Identification of a region in G protein gamma subunits conserved across species but hypervariable among subunit isoforms

The heterotrimeric GTP binding proteins, G proteins, consist of three distinct subunits: alpha, beta, and gamma. There are 12 known mammalian gamma subunit genes whose products are the smallest and most variable of the G protein subunits. Sequencing of the bovine brain gamma(10) protein by electrospray mass spectrometry revealed that it differs from the human protein by an Ala to Val substitution near the N-terminus. Comparison of gamma isoform subunit sequences indicated that they vary substantially more at the N-terminus than at other parts of the protein. Thus, species variation of this region might reflect the lack of conservation of a functionally unimportant part of the protein. Analysis of 38 gamma subunit sequences from four different species shows that the N-terminus of a given gamma subunit isoform is as conserved between different species as any other part of the protein, including highly conserved regions. These data suggest that the N-terminus of gamma is a functionally important part of the protein exhibiting substantial isoform-specific variation.

Targeting of K-Ras 4B by S-trans,trans-farnesyl thiosalicylic acid

Ras proteins regulate cell growth, differentiation and apoptosis. Their activities depend on their anchorage to the inner surface of the plasma membrane, which is promoted by their common carboxy-terminal S-farnesylcysteine and either a stretch of lysine residues (K-Ras 4B) or S-palmitoyl moieties (H-Ras, N-Ras and K-Ras 4A). We previously demonstrated dislodgment of H-Ras from EJ cell membranes by S-trans,trans-farnesylthiosalicylic acid (FTS), and proposed that FTS disrupts the interactions between the S-prenyl moiety of Ras and the membrane anchorage domains. In support of this hypothesis, we now show that FTS, which is not a farnesyltransferase inhibitor, inhibits growth of NIH3T3 cells transformed by the non-palmitoylated K-Ras 4B(12V) or by its farnesylated, but unmethylated, K-Ras 4B(12) CVYM mutant. The growth-inhibitory effects of FTS followed the dislodgment and accelerated degradation of K-Ras 4B(12V), leading in turn to a decrease in its amount in the cells and inhibition of MAPK activity. FTS did not affect the rate of degradation of the K-Ras 4B, SVIM mutant which is not modified post-translationally, suggesting that only farnesylated Ras isoforms are substrates for facilitated degradation. The putative Ras-recognition sites (within domains in the cell membrane) appear to tolerate both C(15) and C(20) S-prenyl moeities, since geranylgeranyl thiosalicylic acid mimicked the growth-inhibitory effects of FTS in K-Ras 4B(12V)-transformed cells and FTS inhibited the growth of cells transformed by the geranylgeranylated K-Ras 4B(12V) CVIL isoform. The results suggest that FTS acts as a domain-targeted compound that disrupts Ras-membrane interactions. The fact that FTS can target K-Ras 4B(12V), which is insensitive to inhibition by farnesyltransfarase inhibitors, suggests that FTS may target Ras (and other prenylated proteins important for transformed cell growth) in an efficient manner that speaks well for its potential as an anticancer therapeutic agent.

Studies of membrane association of CLN3 protein

The product of the CLN3 gene is a novel protein of unknown function. Simulations using amphiphacy algorithms have shown that structurally CLN3 may be another candidate for the family of membranous proteins. Signals controlling intracellular targeting of many membrane proteins are present as short sequences within their cytoplasmic domains. In fact, the sequence of CLN3 protein contains several such signaling sequences, which are conserved among mammals. First, at the N-terminus, potential N-myristoylation motif is present. Second, the C-terminal part of CLN3 protein contains both the dileucine motif, which is a potential lysosomal targeting signal, and the prenylation motif. There is scanty evidence of lysosomal and/or mitochondrial localization of CLN3 protein. However, the question of where the functional site of the cln3 protein exists in vivo remains unanswered. From theoretical calculations, we hypothesized that CLN3 should be an integral part of the membranous micro-environment. First, to test this hypothesis, we initiated detergent-partitioning experiments, localizing CLN3 predominantly in a pool of membranous protein. Further studies have shown that CLN3 protein integrates spontaneously with cellular membranes. Second, based on the prenylation results of CLN3 protein in vitro, we discussed the possible topological consequences of C-terminal fragment of CLN3 protein.

Post-translational modification of human brain type I inositol-1,4,5-trisphosphate 5-phosphatase by farnesylation

In brain, type I inositol-1,4,5-trisphosphate 5-phosphatase (InsP3 5-phosphatase) is the major isoenzyme hydrolyzing the calcium-mobilizing second messenger InsP3. Activity of this enzyme could be measured in both soluble and particulate fractions of tissue homogenates. The protein sequence showed a putative C-terminal isoprenylation site (CVVQ). In this study, two mutants have been generated. The first mutant (C409S) has a serine replacing a cysteine at position 409 of the wild-type enzyme. The second mutant (K407D1) is a deletion mutant that lacks the last five C-terminal amino acids. These constructs were individually expressed by transfection in COS-7 cells. Western blot analysis of wild-type transfected cells indicated that both soluble and particulate fractions had a 43-kDa immunoreactive band, with a higher proportion of the original homogenate associated with the particulate part. On the contrary, when the two mutated constructs were transfected in COS-7 cells, the phosphatase was predominantly soluble. Confocal immunofluorescence studies showed the wild-type enzyme to be present on the cell surface of transfected COS-7 cells and in subcellular compartments around the nucleus. This was not observed for the two mutants, where uniform immunofluorescence labeling was observed throughout the cytosol. Recombinant type I InsP3 5-phosphatase expressed in Escherichia coli was a substrate of purified farnesyltransferase. Altogether, the data therefore suggest a direct participation of Cys-409 in a C-terminally anchored InsP3 5-phosphatase by farnesylation.