The association of heterotrimeric GTP-binding protein (Go) with microtubules

Diverse β subunits of heterotrimeric G proteins are present in thyroid plasma membranes

The functioning of heterotrimeric G protein α subunits in the transduction of hormonal signals to appropriate intracellular responses is well recognized. Much less is known about the distribution of isoforms and functions of G protein β subunits. Here, using specific antibodies, we documented that in plasma membranes of the thyroid cell line Nthy-ori 3-1 all Gβ isoforms-Gβ(1), Gβ(2), Gβ(3), Gβ(4) and Gβ(5) are present, while the Gβ(3) occurs in minute amount. In plasma membrane fraction isolated from pooled postoperative thyroids of patients with nodular goiter and Graves' disease, the Gβ(1), Gβ(2), Gβ(4) and Gβ(5) subunits were found, whereas Gβ(3) could not be detected. Competition studies revealed that the Gβ(2) is the principal Gβ subunit in membranes from cultured thyroid cells, originated from normal thyroid, as well as in membranes from patients' thyroids. This suggests that Gβ(2) subunit cooperates with Gα(s) subunit, the most active of the Gα variants, during stimulation of adenylate cyclase which constitutes the main route of physiological thyroid stimulation.

G alpha selectivity and inhibitor function of the multiple GoLoco motif protein GPSM2/LGN

GPSM2 (G-protein signalling modulator 2; also known as LGN or mammalian Pins) is a protein that regulates mitotic spindle organization and cell division. GPSM2 contains seven tetratricopeptide repeats (TPR) and four Galpha(i/o)-Loco (GoLoco) motifs. GPSM2 has guanine nucleotide dissociation inhibitor (GDI) activity towards both Galpha(o)- and Galpha(i)-subunits; however, a systematic analysis of its individual GoLoco motifs has not been described. We analyzed each of the four individual GoLoco motifs from GPSM2, assessing their relative binding affinities and GDI potencies for Galpha(i1), Galpha(i2), and Galpha(i3) and Galpha(o). Each of the four GPSM2 GoLoco motifs (36-43 amino acids in length) was expressed in bacteria as a GST-fusion protein and purified to homogeneity. The binding of each of the four GST-GoLoco motifs to Galpha(i1)-, Galpha(o)-, and Galpha(s)-subunits was assessed by surface plasmon resonance; all of the motifs bound Galpha(i1), but exhibited low affinity towards Galpha(o). GDI activity was assessed by a fluorescence-based nucleotide-binding assay, revealing that all four GoLoco motifs are functional as GDIs for Galpha(i1), Galpha(i2), and Galpha(i3). Consistent with our binding studies, the GDI activity of GPSM2 GoLoco motifs on Galpha(o) was significantly lower than that toward Galpha(i1), suggesting that the in vivo targets of GPSM2 are most likely to be Galpha(i)-subunits.

Fasciclin II signals new synapse formation through amyloid precursor protein and the scaffolding protein dX11/Mint

Cell adhesion molecules (CAMs) have been universally recognized for their essential roles during synapse remodeling. However, the downstream pathways activated by CAMs have remained mostly unknown. Here, we used the Drosophila larval neuromuscular junction to investigate the pathways activated by Fasciclin II (FasII), a transmembrane CAM of the Ig superfamily, during synapse remodeling. We show that the ability of FasII to stimulate or to prevent synapse formation depends on the symmetry of transmembrane FasII levels in the presynaptic and postsynaptic cell and requires the presence of the fly homolog of amyloid precursor protein (APPL). In turn, APPL is regulated by direct interactions with the PDZ (postsynaptic density-95/Discs large/zona occludens-1)-containing protein dX11/Mint/Lin-10, which also regulates synapse expansion downstream of FasII. These results provide a novel mechanism by which cell adhesion molecules are regulated and provide fresh insights into the normal operation of APP during synapse development.

Proteomics analysis of the neurodegeneration in the brain of tau transgenic mice

Protein tau, a major microtubule-binding protein in the brain, comprises six isoforms generated through alternative mRNA splicing. A dysfunctional form of mutant and normal tau is associated or implicated in the pathogenesis of several neurodegenerative disorders. The neuropathological hallmark of these tau-opathies are intraneuronal depositions of fibrillary aggregates of which neurofibrillary tangles are most common. Several distinct transgene mouse models confirmed that tau protein can cause neurodegeneration directly. This study was aimed at identifying proteins that might play a role in the cellular disturbances caused by overexpression of the longest isoform of human tau in the brain of transgenic mice. We found 34 proteins which differed in integrated intensity by a factor of at least 1.5. These proteins could be sorted into several categories. Some of the phenotypic characteristics found in the htau transgenic mice could be related to proteins found in this study. Several proteins are linked to processes involving apoptosis and neuronal death and have been discussed in papers describing neurodegenerative disorders.