αβ defensin antimicrobial peptide (BPDEF) from the invasive red sea mussel Brachidontes pharaonis (P. Fischer, 1870)

The immune system that plays a major role in determining host fitness in the wild, i.e. under the constraints imposed by ecology and life history. Permanent conflict interactions with the environment are the natural situation for a living creature. To partially resolve this, the immune system evolved and is characterized by an enormous variety of mechanisms and effectors, including the AMPs, although their specificity is poor. In fact, AMPs are universal and extremely successful in dealing with a huge range of pathogens, including bacteria, fungi, protozoa and viruses Amps are oligopeptides composed of varying number of amino acids with a broad spectrum of targeted organisms ranging from viruses to parasites. Some of these compounds have been investigated with a view to possible therapeutic use as an alarming increase of resistance of microorganisms to classical antibiotics has been reported. In the present study, using primers designed from coding sequences from M. galloprovincialis and RACE, the cDNA sequence of the defensin was cloned and characterizated from the Red Sea mussel Brachidontes pharaonis, a Lessepsian invasive species, was first recorded in the Mediterranean seven years after the opening of the Suez Canal in 1869. In the past 30 years it has become abundant in midlittoral and infralittoral rocky habitats, especially along the rocky shores of the Eastern Mediterranean Brachidontes pharaonis has spread as far west as Sicily, probably by ship fouling and generated intensive population. Analysis of the sequence of 262 nucleotides revealed in the B. pharaonis defensine (DefBp) the presence of an ORF coding for 81 amino acids consisting of a signal peptide of 23 aa in the amino terminal portion followed by the mature peptide of 35 a.a. and a carboxy-terminal extension position. The full-length amino acid sequence and the results of the alignment with the sequences of M. galloprovincialis indicate an identity of 93% and a similarity of 96% with the antimicrobial peptide MGD1 and an identity of 78% and a similarity of 84% with the antimicrobial peptide of the same MGD2 Mediterranean mussel. In this species truncated sequences of defensin, including amino acid replacement to maintain 3D structure and increased positive net charge, possess antibacterial, antiprotozoan and antiviral capabilities. HIS analysis revealed BpDef mRNA expressed in circulating hemocytes with small intra-cytoplasmic granules (B, D) and with large granules Sequence identity and the common conserved sequence characteristics show that BpDef belongs to the  defensin family of AMPs with a typical domain structurally characterized by a helix and two sheets  . Two predicted bactericidal stretches were found in correspondence of the mature peptide and the Boman index value indicated the strong antibacterial feature of defensin of B. pharaonis.