Distribution and role of heterotrimeric G proteins in the secretory pathway of polarized epithelial cells.

The movement of newly synthesized proteins in the constitutive secretory pathway, from their site of synthesis in the endoplasmic reticulum to the cell surface or to intracellular destinations, requires an orderly sequence of transport steps between membrane-bound compartments. Until recently, the trafficking and secretion of proteins through this pathway was thought to occur as a relatively automatic, unregulated series of events. Recent studies show that protein trafficking in the constitutive secretory pathway requires GTP hydrolysis by families of GTP-binding proteins (G proteins), which at multiple steps potentially provide regulation and specificity for protein trafficking. Many monomeric G proteins are known to be localized and functional on membrane compartments in the constitutive secretory pathway. Now, members of the heterotrimeric G protein family have also been localized on intracellular membranes and compartments such as the Golgi complex. We have studied the localization and targeting of G alpha subunits to distinct membrane domains in polarized epithelial cells. The distribution of different G alpha subunits on very specific membrane domains in cultured epithelial cells and in epithelial cells of the kidney cortex, is highly suggestive of roles for these G proteins in intracellular trafficking pathways. One of these G protein subunits, G alpha i-3, was localized on Golgi membranes. Studies on LLC-PK1 cells overexpressing G alpha i-3 provided evidence for its functional role in regulating the transport of a constitutively secreted heparan sulfate proteoglycan through the Golgi complex. Inhibition or activation of heterotrimeric G proteins by pertussis toxin or by aluminium fluoride respectively, have provided further evidence for regulation of intracellular transport by pertussis toxin-sensitive G proteins.(ABSTRACT TRUNCATED AT 250 WORDS)