Evidence for an unusual transmembrane configuration of AGG3, a class C Gγ subunit of Arabidopsis

Heterotrimeric G proteins are crucial for the perception of external signals and subsequent signal transduction in animal and plant cells. In both model systems, the complex comprises one Gα, one Gβ, and one Gγ subunit. However, in addition to the canonical Gγ subunits (class A), plants also possess two unusual, plant-specific classes of Gγ subunits (classes B and C) that have not yet been found in animals. These include Gγ subunits lacking the C-terminal CaaX motif (class B), which is important for membrane anchoring of the protein; the presence of such subunits gives rise to a flexible sub-population of Gβ/γ heterodimers that are not necessarily restricted to the plasma membrane. Plants also contain class C Gγ subunits, which are twice the size of canonical Gγ subunits, with a predicted transmembrane domain and a large cysteine-rich extracellular C-terminus. However, neither the presence of the transmembrane domain nor the membrane topology have been unequivocally demonstrated. Here, we provide compelling evidence that AGG3, a class C Gγ subunit of Arabidopsis, contains a functional transmembrane domain, which is sufficient but not essential for plasma membrane localization, and that the cysteine-rich C-terminus is extracellular.

[BSCL2-related neurologic disorders/seipinopathy: endoplasmic reticulum stress in neurodegeneration]

The Seipin/BSCL2 gene was originally identified as a loss-of-function gene for congenital generalized lipodystrophy type 2, a condition characterized by severe lipoatrophy, insulin resistance, and hypertriglyceridemia. Whereas gain-of-toxic-function mutations (namely, mutations N88S and S90L) in the seipin gene have been identified in autosomal dominant motor neuron diseases such as Silver syndrome/spastic paraplegia 17 (SPG17) and distal hereditary motor neuropathy type V. Detailed phenotypic analyses have revealed that upper motor neurons, lower motor neurons and peripheral motor axons are variously affected in patients with these mutations. We recently showed that the N88S and S90L mutations disrupt the N-glycosylation motif, enhance ubiquitination, and appear to result in proteins that are improperly folded, leading to accumulation of the mutant protein in the endoplasmic reticulum (ER). We also showed that expression of mutant in cultured cells activates the UPR pathway and induces cell death, suggesting that seipinopathy is tightly associated with ER stress, which has recently been reported to be associated with other neurodegenerative diseases. Further study of the pathological mechanisms of the mutant forms of seipin may lead to important new insights into motor neuron diseases, including other spastic paraplegia diseases and amyotrophic lateral sclerosis.

Dual lipid modification of Arabidopsis Ggamma-subunits is required for efficient plasma membrane targeting

Posttranslational lipid modifications are important for proper localization of many proteins in eukaryotic cells. However, the functional interrelationships between lipid modification processes in plants remain unclear. Here we demonstrate that the two heterotrimeric G-protein gamma-subunits from Arabidopsis (Arabidopsis thaliana), AGG1 and AGG2, are prenylated, and AGG2 is S-acylated. In wild type, enhanced yellow fluorescent protein-fused AGG1 and AGG2 are associated with plasma membranes, with AGG1 associated with internal membranes as well. Both can be prenylated by either protein geranylgeranyltransferase I (PGGT-I) or protein farnesyltransferase (PFT). Their membrane localization is intact in mutants lacking PFT activity and largely intact in mutants lacking PGGT-I activity but is disrupted in mutants lacking both PFT and PGGT-I activity. Unlike in mammals, Arabidopsis Ggammas do not rely on functional Galpha for membrane targeting. Mutation of the sixth to last cysteine, the putative S-acylation acceptor site, causes a dramatic change in AGG2 but not AGG1 localization pattern, suggesting S-acylation serves as an important additional signal for AGG2 to be targeted to the plasma membrane. Domain-swapping experiments suggest that a short charged sequence at the AGG2 C terminus contributes to AGG2's efficient membrane targeting compared to AGG1. Our data show the large degree to which PFT and PGGT-I can compensate for each other in plants and suggest that differential lipid modification plays an important regulatory role in plant protein localization.

G protein βγ subunits interact with αβ- and γ-tubulin and play a role in microtubule assembly in PC12 cells

The betagamma subunit of G proteins (Gbetagamma) is known to transfer signals from cell surface receptors to intracellular effector molecules. Recent results suggest that Gbetagamma also interacts with microtubules and is involved in the regulation of the mitotic spindle. In the current study, the anti-microtubular drug nocodazole was employed to investigate the mechanism by which Gbetagamma interacts with tubulin and its possible implications in microtubule assembly in cultured PC12 cells. Nocodazole-induced depolymerization of microtubules drastically inhibited the interaction between Gbetagamma and tubulin. Gbetagamma was preferentially bound to microtubules and treatment with nocodazole suggested that the dissociation of Gbetagamma from microtubules is an early step in the depolymerization process. When microtubules were allowed to recover after removal of nocodazole, the tubulin-Gbetagamma interaction was restored. Unlike Gbetagamma, however, the interaction between tubulin and the alpha subunit of the Gs protein (Gsalpha) was not inhibited by nocodazole, indicating that the inhibition of tubulin-Gbetagamma interactions during microtubule depolymerization is selective. We found that Gbetagamma also interacts with gamma-tubulin, colocalizes with gamma-tubulin in centrosomes, and co-sediments in centrosomal fractions. The interaction between Gbetagamma and gamma-tubulin was unaffected by nocodazole, suggesting that the Gbetagamma-gamma-tubulin interaction is not dependent on assembled microtubules. Taken together, our results suggest that Gbetagamma may play an important and definitive role in microtubule assembly and/or stability. We propose that betagamma-microtubule interaction is an important step for G protein-mediated cell activation. These results may also provide new insights into the mechanism of action of anti-microtubule drugs.

Immunohistochemical localization of G protein betagamma subunits in the lateral wall of the rat cochlea

The role of G protein-mediated signal transduction in the production of endolymph, an extracellular fluid of unusual ionic composition, is beginning to be understood. The identity of Galpha subunits in the stria vascularis and the spiral ligament of the lateral wall of the cochlear duct is well established. However, little is known about the presence of betagamma subunits. This study used immunohistochemistry to investigate the distribution of G protein betagamma subunits in the lateral wall of the cochlea. Temporal bones of 6- to 8-week-old rats were fixed in 4% paraformaldehyde and 0.1% glutaraldehyde and processed for embedding in paraffin wax. The dewaxed, midmodiolar sections of the cochlea were incubated with subunit-specific polyclonal antibodies. The results show that the pattern of immunoreactivity varies for the G protein beta1-4 and gamma1-3, 5 and 7 subunits in the stria vascularis and spiral ligament. In the stria vascularis, immunoreactivity was detected for beta2, beta3, beta4, gamma1, gamma2 and gamma7 subunits. All five types of fibrocytes in the spiral ligament exhibited positive staining for gamma2 and gamma7. However, immunoreactivity for beta1-4 subunits was variable. Immunoreactivity for gamma3 and gamma5 subunits was not detected in the lateral cochlear wall. The expression pattern of G protein betagamma subunits in lateral wall provides a basis for interpreting the functions of G protein-coupled receptors in cochlear fluid homeostasis.

The phosducin-like protein PhLP1 is essential for G{beta}{gamma} dimer formation in Dictyostelium discoideum

Phosducin proteins are known to inhibit G protein-mediated signaling by sequestering Gbetagamma subunits. However, Dictyostelium discoideum cells lacking the phosducin-like protein PhLP1 display defective rather than enhanced G protein signaling. Here we show that green fluorescent protein (GFP)-tagged Gbeta (GFP-Gbeta) and GFP-Ggamma subunits exhibit drastically reduced steady-state levels and are absent from the plasma membrane in phlp1(-) cells. Triton X-114 partitioning suggests that lipid attachment to GFP-Ggamma occurs in wild-type cells but not in phlp1(-) and gbeta(-) cells. Moreover, Gbetagamma dimers could not be detected in vitro in coimmunoprecipitation assays with phlp1(-) cell lysates. Accordingly, in vivo diffusion measurements using fluorescence correlation spectroscopy showed that while GFP-Ggamma proteins are present in a complex in wild-type cells, they are free in phlp1(-) and gbeta(-) cells. Collectively, our data strongly suggest the absence of Gbetagamma dimer formation in Dictyostelium cells lacking PhLP1. We propose that PhLP1 serves as a cochaperone assisting the assembly of Gbeta and Ggamma into a functional Gbetagamma complex. Thus, phosducin family proteins may fulfill hitherto unsuspected biosynthetic functions.