Co-Localization by Immunofluorescence of the α Subunit(S) of Gi with Cytoplasmic Structures
GTP-binding proteins established in signal transduction (G proteins) are hetemtrimers consisting of α, β, and γ subunits. From the initial discovery Gs, the family of G proteins has expanded in both number and scope of action. Arrangements of G proteins within the plasma membrane and among subcellular structures, however, remain largely uncharacterized. Compartmentation within the plasma membrane may indeed provide the means by which agonists can operate selectively on particular enzymes or channels (Brass et al., 1988; Neer et al., 1988). G proteins have also been detected within the cytosol (Bokoch et al., 1988; Rotrosen et al., 1988; Spicher et al., 1988) and in granule membranes (Toutant et al., 1987; Rotrosen et al., 1988). While cytoplasmic populations of G proteins may simply represent internal stores bound for recruitment to the plasma membrane, there is evidence that they may perform more active roles critical to normal cell function. The existence of guanine nucleotide-sensitivity, for example, has been demonstrated in transport of protein within the Golgi apparatus (Melançon et al., 19871, degranulation (Cockroft et al., 1987; Lindau and Nusse,1987), release of Ca++ from endoplasmic reticulum (Nasmith and Grinstein, 1987), and perhaps translocation of nascent proteins (Audigier et al., 1988; Connolly and Gilmore, 1986; Hoffman and Gilmore, 1988). In parallel to these findings, a recent study has shown that a 41-kDa, Giα-like protein co-fractionates with rough endoplasmic reticulum (Audigier et al, 1988). Clearly, knowledge of the distributions of G proteins within the cell will be critical to our understanding of G protein function.
KeywordsLymphoma Tyrosine MgCl2 Fractionation Cytosol
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