Protein the structure and function of proteins.

Protein
sequence alignment is one of the significant techniques of computational
biology which shapes the groundwork of many other procedures like phylogenetic
analysis, prediction of the structure and function of proteins. In the current
study, insilico analyses of avian LEAP2 sequences were performed. Mature
peptide region were found to be highly conserved than prodomain and signal
sequence. Majority of the selected birds possessed RQRR motif at the cleavage
site between proregion and mature region. Functional prediction revealed that only
LEAP2 of Aquila
chrysaetos Canadensis plays some role in signaling pathway, oocyte
and follicular development, cytokine activity and apoptosis regulation beyond
microbial killing.  Higher conservation
in the amino acid sequences of LEAP2 among birds suggests the significant role
of this molecule in avian evolution.

Introduction

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Antimicrobial
peptides (AMPs) are crucial elements of the host non-specific innate immune
system which play a major role as natural antibiotics by providing first line
defense against microbial incursion. (Hancock et al. 2006; Zasloff et al.
2002). AMPs are generally short, cationic, amphipathic and structurally diverse
molecules that are widely present in nearly all life form. These molecules
display extensive and strong antimicrobial activity against microbial assault
primarily by destroying membrane integrity of the invading pathogens. (Mansour et al. 2014; Nijnik et al. 2009). Apart from being
antimicrobial, AMPs execute immunomodulatory functions also by activating
innate and adaptive immune cells; hence they were named as host defense
peptides (HDPs) (Yacoub et al. 2015).  Majority of the AMPs are encoded by specific
genes, and numerous structurally diverse HDPs usually present in a single
species. (Zhang et al. 2014).Different
groups of AMPs reported in birds include avian ?-defensin (AvBD),  cathelicidin, liver-expressed antimicrobial
peptide 2 (LEAP2) and NK-lysin which endow them with tremendous innate immunity (Ishige et al. 2016).

Liver-expressed
antimicrobial peptide 2 (LEAP-2) is the second blood-derived peptide identified
firstly from human blood and shows antimicrobial activity and predominant
expression in the liver. This peptide
belongs to cystein rich family of AMPs characterized by the presence of a four
cystein conserved motif and two intracellular disulphide bridges (Krause et al. 2003).  LEAP2 has been
characterized from few organisms like human, guinea pig,
fishes, monkey, pig,
mouse, cow, and chicken (Townes et al.
2004; Krause et
al. 2003; Liang et al. 2013; Ren et al. 2014). In chicken, LEAP2
gene consists
of three exons and two introns; has been reported to be localized in chromosome 13 of
chicken genome (Michailidis. 2010).
Like other cationic HDP, LEAP2 also possess a signal peptide, a prepropeptide
and an active mature peptide (Steinstraesser
et al. 2011). The RXXR motif at cleavage site between prodomain and mature
region of LEAP2 is conserved throughout vertebrates (Zhang et al. 2004).

Chicken LEAP2 (cLEAP-2) gene encodes a 76 amino acid prepropeptide which is enzymatically
processed by furin family of propeptide convertase to release mature peptide of
40 amino acid. Primary structure analysis of cLEAP2 specified the presence of
RLKR motif ahead of the cleavage site separating the proregion
and the mature peptide. The expression of cleap2 in
different epithelial tissues suggests its role in preventing the microbes from
interacting with epithelial surfaces and tissue invasion. cLEAP-2
has been reported to exhibit antimicrobial activity against Salmonella
spp., Streptococcus spp. and Staphylococcus spp (Townes et al. 2004; Townes et al.
2009). Studies suggest that in chicken, human and
several species of fishes LEAP2 gene express several splicing variants as a
result of alternative splicing (Townes et al. 2004; Howard et al.
2010; Li et al. 2014). It is possible that LEAP2
gene employs splicing as a major mechanism of its regulation. Howard et al (2010)
reported that alternative splicing of human LEAP2 in diverse gastro-intestinal
tissues generates splice variants predicted to be both secretory and
intracellularly positioned peptides. The occurrence of intracellular variants
of LEAP2, devoid of signal sequence recommends that LEAP2 may also have
additional functions other than antimicrobial activity.

Multiple sequence alignment (MSA) is
an indispensable tool in biological sequence analysis that aids
to establish evolutionary distance and phylogenetic relationship among
organisms. The
detection of similar motifs in evolutionarily linked proteins would help in the
prediction of structurally or functionally significant positions (Chaabane et al. 2015; Vijan et al. 2011; Do et al. 2008). In the present
study, we aligned the LEAP2 protein sequences of birds including chicken which
were retrieved from NCBI to reveal the similarities and differences of amino
acid residues of LEAP2 across the group. Elucidation of the possible functional
attribute of LEAP2 was also performed using BLAST2GO software.

 

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