The human cathelicidin LL-37 displays both direct antibacterial
activities and the capacity to modulate host-cell activities. These
depend on structural characteristics that are subject to positive
selection for variation, as observed in a previous analysis of
the CAMP gene (encoding LL-37) in primates. The altered
balance between cationic and anionic residues in different primate
orthologues affects intramolecular salt-bridging and influences
the stability of the helical conformation and tendency to aggregate
in solution of the peptide. In the present study, we have analysed
the effects of these structural variations on membrane interactions
for human LL-37, rhesus RL-37 and orang-utan LL-37, using
several complementary biophysical and biochemical methods.CD
and ATR (attenuated total reflection)-FTIR (Fourier-transform
IR) spectroscopy on model membranes indicate that RL-37,
which is monomeric and unstructured in bulk solution [F-form
(free form)], and human LL-37, which is partly structured and
probably aggregated [A-form (aggregated form)], bind biological
membranes in different manners. RL-37 may insert more deeply
into the lipid bilayer than LL-37, which remains aggregated.
AFM(atomic forcemicroscopy) performed on the same supported
bilayer as used for ATR-FTIR measurements suggests a carpetlike
mode of permeabilization for RL37 and formation of more
defined worm-holes for LL-37. Comparison of data from the biological
activity on bacterial cells with permeabilization of model
membranes indicates that the structure/aggregation state also
affects the trajectory of the peptides from bulk solution through
the outer cell-wall layers to the membrane. The results of the
present study suggest that F-form cathelicidin orthologues may
have evolved to have primarily a direct antimicrobial defensive
capacity, whereas the A-forms have somewhat sacrificed this to
gain host-cell modulating functions.
