Molecular Characterization and Gene Expression Profiling ... - CUSAT

Chapter 1
lipid head groups (Matsuzaki, et al., 1996). This type of transmembrane pore
is induced by magainins, protegrins and melittin (Matsuzaki et al., 1996;
Yang et al., 2001; Hallock et al., 2003). In forming a toroidal pore, the polar
faces of the peptides associate with the polar head groups of the lipids
(Yamaguchi et al., 2002). The lipids in these openings then tilt from the
lamellar normal and connect the two leaflets of the membrane, forming a
continuous bend from the top to the bottom in the fashion of a toroidal hole;
the pore is lined by both the peptides and the lipid head groups, which are
likely to screen and mask cationic peptide charges (Yang et al., 2001). The
toroidal model differs from the barrel-stave model as the peptides are always
associated with the lipid head groups even when they are perpendicularly
inserted in the lipid bilayer (Yang et al., 2001). Magainin-induced toroidal
pores are larger and have a more variable pore size than alamethicin-
induced pores (Yang et al., 2001). They have an inner diameter of 3.0–5.0 nm
and an outer diameter of ~7.0–8.4 nm, and each pore is thought to contain
only 4–7 magainin monomers and ~90 lipid molecules (Matsuzaki et al.,
1997, 1998; Yang et al., 2001). The mechanisms of defensins are not as well
defined (Zasloff, 2002; Lehrer, 2004) but they also permeabilize membrane
bilayers containing negatively charged phospholipids (Fujii et al., 1993;
Zasloff, 2002; Lehrer, 2004). Although descriptions of membrane damage
seem to vary, they are likely to be related. It has been suggested that ion
channels, transmembrane pores and extensive membrane rupture do not
represent three completely different modes of action, but instead are a
continuous gradation between them (Dathe and Wieprecht, 1999).
1.7.10.4 Models of intracellular killing
Although the formation of ion channels, transmembrane pores and
extensive membrane rupture eventually leads to the lysis of microbial cells,
there is increasing speculation that these effects are not the only mechanisms
of microbial killing. There is increasing evidence to indicate that AMPs have
Molecular Characterization and Gene Expression Profiling of Antimicrobial Peptides in Penaeid Shrimps
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