STRUCTURE/FUNCTION MODELLING STUDIES OF HTLV-I SURFACE PROTEIN (SU)

Abstract

P006 For HTLV-I, the process of viral entry is poorly understood, and the cellular receptor(s) have not been identified. Little is known about the overall structure or the receptor-binding regions of the HTLV-I SU. Several regions that may be involved in receptor interactions were predicted from models based on sequence alignments of HTLV-I SU with the receptor-binding domains of FMuLV and amphotropic MuLV SUs. One region identified, with some homology to the variable region B (VRB) of the MuLV SUs, includes the location of 2 mutants (R94Q, S101L) previously shown by one of us (M.-C. D.) to effect syncytia formation and cell-to-cell transmission. Studies of S101L/HTLV-I virus have revealed that SU is incorporated into virions, but the mutant virus is much less infectious than WT virus. This suggests that this mutation effects binding or postbinding events. To better understand the role of this and other regions identified by the alignments described above, we have introduced additional single amino acid changes in conserved residues in these regions. We have examined the effect of mutations on structure directly using monoclonal antibodies that recognize conformational epitopes of HTLV-I SU. When expression of two independent conformational epitopes was examined, evidence for an ∼2.5 fold reduction in levels of cell surface expression of SU carrying mutations in R94, S101, and Y104 relative to WT SU was obtained. These results suggest that the effect of these mutations on the life cycle of HTLV-I may reflect both reduced concentration of conformationally intact SU and functional impairment.