DISTRIBUTION AND ROLE OF HETEROTRIMERIC G-PROTEINS IN THE SECRETORY PATHWAY OF POLARIZED EPITHELIAL-CELLS

The movement of newly synthesized proteins in the constitutive secreto ry pathway, from their site of synthesis in the endoplasmic reticulum to the cell surface or to intracellular destinations, requires an orde rly sequence of transport steps between membrane-bound compartments. U ntil recently, the trafficking and secretion of proteins through this pathway was thought to occur as a relatively automatic, unregulated se ries of events. Recent studies show that protein trafficking in the co nstitutive secretory pathway requires GTP hydrolysis by families of GT P-binding proteins (G proteins), which at multiple steps potentially p rovide regulation and specificity for protein trafficking. Many monome ric G proteins are known to be localized and functional on membrane co mpartments in the constitutive secretory pathway. Now, members of the heterotrimeric G protein family have also been localized on intracellu lar membranes and compartments such as the Golgi complex. We have stud ied the localization and targeting of G alpha subunits to distinct mem brane domains in polarized epithelial cells. The distribution of diffe rent G alpha subunits on very specific membrane domains in cultured ep ithelial cells, and in epithelial cells of the kidney cortex, is highl y suggestive of roles for these G proteins in intracellular traffickin g pathways. One of these G protein subunits, G alpha(i-3), was localiz ed on Golgi membranes. Studies on LLC-PK1 cells overexpressing G alpha (i-3) provided evidence for its functional role in regulating the tran sport of a constitutively secreted heparan sulfate proteoglycan throug h the Golgi complex. Inhibition or activation of heterotrimeric G prot eins by pertussis toxin or by aluminium fluoride respectively, have pr ovided further evidence for regulation of intracellular transport by p ertussis toxin-sensitive G proteins. Although the functions of Golgi-a ssociated G proteins are not yet understood at the molecular level, he terotrimeric G proteins have been implicated in the binding of cytosol ic coat proteins and vesicle formation on Golgi membranes. Future stud ies will elucidate how multiple G proteins, of both the heterotrimeric and monomeric families, are involved in the regulation of Golgi funct ion and protein trafficking in the secretory pathway. subunits on very specific membrane domains in cultured epithelial