Background: Sheath or gelling saliva, secreted during feeding by aphids, is a hard material that
supports the piercing mouthparts and remains in the plant after feeding. Solidification or gelling of the saliva
might be due to the composition of amino acids in the constituent proteins, many of which probably interact
with plant defenses.
Objective: The complete complement of proteins in the gelling saliva are still unknown, although one sheath
protein (SHP) has previously been identified as a potential candidate protein to control aphid feeding, but its
structure and its physiochemical role remains obscure. The current study provides structural information and
biochemical properties of the aphid sheath protein.
Methods: The Sheath protein encoding gene was amplified from cDNA of the pea aphid (Acyrthosiphon pisum)
through PCR using specific gene primers. Sequence was in silico characterized by using EXPASY, Berkeley
Drosophila Genome Project (BDGP) Neural Network Promoter Prediction, BioEdit, Mega7, ProtParam, Phyre
server, 3D LigandSite SMART, MEME and GSDS programs, available online.
Results: BLASTp analysis revealed that the sequenced gene was identical (100%) to the sequence from
Acyrthosiphon pisum, with 87% identity to Metpolophium dirhodum and 84% identity to Sitobion avenae.
Phylogenetically monocot feeders such as M. dirhodum and S. avenae are in a sister taxa to dicot feeders. In
silico analysis of the sequence revealed that sheath protein has a molecular weight of 144 kDa and 50% of the
protein is composed of only six amino acids, i.e., threonine, serine, aspartic acid, glutamic acid, isoleucine and
tyrosine. The computed IP value revealed that sheath protein is acidic in nature. Ligand binding sites for sheath
protein were predicted on residues 1123 and 1125 (isoleucine and glutamine, respectively). Metallic heterogens
are also present in sheath protein that are iron, zinc and magnesium, respectively.
Conclusion: It is conceivable that variation in the salivary gene sequences may reveal important biological
information of relevance to the insect-plant interaction. Further exploration of insect salivary proteins, their
composition and structure will provide powerful information, especially when these proteins are interacting with
plant proteins, and specific information about the sheath protein, which is interacting with plants at a
molecular/cellular level, will be important to progress strategies aimed specifically against sucking pests such as