Large Changes In The Crac Segment Of Gp41 Of Hiv Do Not Destroy Fusion Activity If The Segment Interacts With Cholesterol

The membrane-proximal external region (MPER) of the gp41 fusion protein of HIV is
highly conserved among isolates of this virus and is considered a target for
vaccine development. This region also appears to play a role in membrane fusion
as well as localization of the virus to cholesterol-rich domains in membranes.
The carboxyl terminus of MPER has the sequence LWYIK and appears to have an
important role in cholesterol interactions. We have tested how amino acid
substitutions that would affect the conformational flexibility of this segment
could alter its interaction with cholesterol. We studied a family of peptides
(all peptides as N-acetyl-peptide amides) with P, G, or A substituting for W and
I of the LWYIK sequence. The peptide having the greatest effect on cholesterol
distribution in membranes was the most flexible one, LGYGK. The corresponding
mutation in gp41 resulted in a protein retaining 72% of the fusion activity of
the wild-type protein. Two other peptides were synthesized, also containing two
Gly residues, GWGIK and LWGIG, and did not have the ability to sequester
cholesterol as efficiently as LGYGK did. Making the corresponding mutants of gp41
showed that these other two double Gly substitutions resulted in proteins that
were much less fusogenic, although they were equally well expressed at the cell
surface. The study demonstrates that drastic changes can be made in the LWYIK
segment with the retention of a significant fraction of the fusogenic activity,
as long as the mutant proteins interact with cholesterol.