**1.6 Tachyplesin**

This class of AMP was first found in horseshoe crab (*Polyphemus litoralis*). Gomesin is a tachyplesin type of antimicrobial peptide found in tarantula hemocyte (Silva et al., 2000); and androctonin has been extracted from scorpion hemolymph (Mandard et al., 1999). This type of antimicrobial structure is broadly distributed amongst the genus. The AMP related to this class of peptide present a beta sheet secondary structure stabilized by two disulfide bridges (Nakamura et al., 1988). Tachyplesin have a rigid conformation of antiparallel betasheet connected by a beta-turn (Iwanaga et al., 1994). The tachyplesin family of AMP adopts

Natural Antimicrobial Peptides from Eukaryotic Organisms 57

terminus, this peptide could effectively block these channels. Even though, the antimicrobial activity of the complete molecule was dependent of the presence of this toxin/defensin

The penaeidin class of peptide consist in proline-rich N-terminus and of a C-terminus containing six cysteine residues engaged in three disulfide bridges (Destoumieux et al., 2000). The proline-rich domain of penaeidin class AMP suffices to confer target specificity and antimicrobial activity of penaeidin (Cuthbertson et al., 2004). The carboxyl end cysteinerich domain consists of an amphipathic helix linked to the upstream and the downstream coils by two disulfide bonds. The peptide shows a highly hydrophobic core of globular and

Another example of hybrid antimicrobial peptide is Hyastatin, isolated from the spider crab (*Hyas araneus*) hemocytes (Cuthbertson et al., 2008). This AMP combines a Glycine rich motif N-terminal region, a short Pro/Arg-rich region, and a panaeidin like C-terminal region

The chicken beta defensin 11 is formed by the repeat of two defensin motif, therefore having 6 disulfid bridges. This defensin show a nanomolar range of anti *E.coli* activity, being one of the most effective antimicrobial peptide for this microorganism (Herve-Grepinet et al., 2010).

Microplusin, is a *Rhipicephalus* (*Boophilus*) microplus anti-microbial peptide (AMP). Microplusin has a cysteine-rich AMPs structure with histidine-rich regions at the N- and Ctermini. Microplusin consists of five alpha-helix and has been shown to bind copper and

The activity of AMPs must start at the cytoplasmic membrane since most AMPs permeabilize microbial membranes. Several models have been proposed on how AMPs insert into the membrane leading to the formation of ion channels, transmembrane pores or extensive membrane rupture. These models are: 1) transmembrane pore models and 2) nonpore models activity. Here we will also review other antimicrobial mechanisms that have been found. For example, the antimicrobial mechanisms of apidaecin do not rely on pore forming activity, this peptide does have antimicrobial activity at a concentration at least four order of magnitude below the concentration that disturbs the bacterial membrane. Peptides from each structural family have been reported to rely on antimicrobial mechanism that would not imply membrane depolarization of the target microorganism, suggesting internal molecular determinant. Certain peptides are unable to cause membrane depolarization at the minimal inhibitory concentration, while other cause maximal depolarization well below the MIC (Minimal Inhibitory Concentration) value. Evidences are mounting that involve particular macromolecules as well as intracellular functions as the final target for antimicrobial activity of the AMP. Even though, bacterial membranes are a necessary entry path for the AMP,

There are more than 1,000 known AMPs (Brahmachary et al., 2004; Wang and Wang, 2004; Fjell et al., 2007), and for the majority of them, there is little or no evidence for

therefore determining part of the AMP selectivity as well as efficiency.

compact structure, that has 2 arginines exposed on each side (Yang et al., 2003).

motif (Diego-Garcia et al., 2008).

iron (Silva et al., 2009).

containing 3 disulfid bridges (Sperstad et al., 2009).

**2. Antimicrobial mechanisms of AMP** 

**2.1 Transmembrane pore models of AMPs** 

beta-hairpin-like structures when in contact with hydrophobic solvent. NMR studies revealed a largely unordered structure in water, but a transition to a regular beta-hairpin backbone conformation in the presence of dodecylphosphocholine micelles. The cysteine null mutant of protegrin, a mammal tachyplesin type peptide, revealed that the cystein bridges were not necessary for antimicrobial activity. Aside from their antimicrobial activity, tachyplesin have also a scavenger capability. They bind lipo-polysacharide with high affinity (Niwa et al., 1990). The structure of tachyplesin I also interacts with Vesicular stomatitis virus envelope, inactivating the virus (Murakami et al., 1991).

Tachyplesin III *Tachypleus gigas*: KWCFRVCYRGICYRKCR
