Characterization of an aphid-specific, cysteine-rich protein enriched in salivary glands.

Aphids secrete saliva into the phloem during their infestation of plants. Previous studies have identified numerous saliva proteins, but little is known about the characteristics (physical and chemical) and functions of these proteins in aphid-plant interactions. This study characterized an unknown protein (ACYPI39568) that was predicted to be enriched in the salivary glands of pea aphid. This protein belongs to an aphid-specific, cysteine-rich protein family that contains 14 conserved cysteines. ACYPI39568 is a monomeric globular protein with a high beta strand extent. The binding stoichiometric ratios for Zn(2+) and ACYPI39568 were approximately 3:1 and 1:1 at two binding sites. ACYPI39568 was predominantly expressed in the first instar stage and in the salivary glands. Aphids required more ACYPI39568 when feeding on plants than when feeding on an artificial diet. However, the interference of ACYPI39568 expression did not affect the survival rate of aphids on plants.