The vesicular stomatitis virus glycoprotein G is responsible for low pH mediated membrane fusion induced by the virus. Four linker insertion mutants (H2, H5, HIO, A4) of the G ectodomain were found to disrupt fusion and yet maintained all the requirements for proper folding and cell surface expression (Li et al., 1993). Site specific mutagenesis of residues 123 to 137, surrounding the H2 mutant, either blocked or shifted the pH optima and threshold of fusion to more acidic values with a concomitant reduction in cell-cell fusion efficiency (Zhang and Ghosh, 1994; Fredericksen and Whitt, 1995). The region is highly conserved among vesiculoviruses and was found to insert into lipid membranes by hydrophobic photolabelling (Durrer et al., 1995) suggesting a possible role for this domain as the fusion peptide. Site-directed mutagenesis of residues 190 to 210, surrounding the H5 insertion mutant, did not significantly affect fusion (Fredericksen and Whitt, 1995). Surrounding the H10 and A4 insertion mutants is a conserved region, residues 395 to 424, that does not interact with target membranes (Durrer et al., 1995). To determine the functional importance of this region, site-directed mutagenesis was employed. Substitution of conserved Gly 404, Gly 406, Asp 409, and Asp 411 with Ala, Ala, Asn, and Asn, respt:.ctively, both reduced fusion and caused a shift in the pH of fusion threshold to more acidic values (tested by Y. He as published in Shokralla et al., 1998). In this study, the Hl0/A4 region is further mutagenized and tested for fusion. Cell surface expression was examined by indirect immunofluorescence and lactoperoxidase catalyzed iodination. Rates of transport from the endoplasmic reticulum and oligomerization into trimers were tested by resistance to endoglycosidase H and sucrose density gradient centrifugation, respectively. Low-pH induced conformational changes were assayed by resistance to proteolytic digestion. Residues Gly 395, Gly 404, Gly 409 and Ala 418 were substituted with Glu, Lys, Asp, and Lys, respectively. All mutants, with the exception of A418K, were expressed at levels similar to or above wild-type. Mutants G404K and D409A completely abolished fusion. Mutant G395E reduced cell-cell fusion efficiency by 82% and shifted both the pH threshold and optimum of wild type fusion. Although all mutants were capable of trimer formation, alterations in the structure of mutants G404K, D409 A, and A418K were detected by slower transport rates. All Hl0/A4 mutants were more susceptible to trypsin than wild-tyr,e at the pH of6.5, and mutant G404K was completely susceptible at this pH Reductions in the extent of fusion, along with shifts in the pH optima and thresholds of fusion suggest that the Hl0/A4 region (residues 395 to 418) of vesicular stomatitis virus G protein is important for G mediated fusion. The region may influence low-pH induced conformational changes. Double mutants of the H2 and HI0/A4 regions were also tested for their effects on fusion. The extents of fusion mediated by double mutant G proteins were severely reduced with levels ranging from 28% wild-type fusion to complete fusion deficiency. Only mutant Gl31A G404A was capable of 83% wild-type fusion. Mutants Gl31A G395E, Gl31A G404A, Gl31A D4LIN, Dl37N G404A, and the fusion defective D137N D411N were expressed at levels above wild-type G protein at the cell surface. Mutants Fl25Y D411N and Pl26L D411N, although capable of very low levels of fusion were not detectable at the cell surface by immunoflorescence and were detected at low levels by lactoperoxidase catalyzed iodination of cell surface proteins. These two mutants, along with Gl31A G404A, also showed slower transport rates than wild-type G. All double mutants showed increased sensitivity to trypsin at the pH of 6.5 with mutant Fl25Y D411N showing complete susceptibility. They were also all capable of trimer formation by sucrose density gradient centrifugation. In comparing the fusion profiles of double mutants with those of their component single mutants, it was found that in most cases the pH threshold of fusion by double mutants was greater than the sum of the single mutants and that the pH optimum of fusion corresponded to that of the constituent H2 single mutant. Although, the regions are functionally independent, they may indirectly affect one another through alterations in protein structure. / Thesis / Master of Science (MS)