Abstract: The apidermin (APD) protein family is a novel structural cuticular protein family of insects. To gain a better understanding of this protein family, by using bioinformatics and RT-PCR amplification, we identified three novel apd genes (apd-1-like, apd-3-like from Apis mellifera ligustica, and apd-2 from Apis cerana cerana) and investigated their structural features, and then we revealed the characteristic motifs of the APD family. The results showed that a cluster of six apd genes were tandemly arranged on chromosome 4 of Apis mellifera. These apd genes were differentially expressed in drone head of A. m. ligustica, and their expression pattern was consistent with that of the cis-acting elements in their promoter sequences. The genomic DNA of apd-2 from A .c. cerana and apd-1-like from A. m. ligustica contain three exons and two introns, while that of apd-3-like from A. m. ligustica contain four exons and three introns. As the deduced proteins of the apd genes were analyzed, it was found that the presently available 10 APDs possess similar N-terminal signal peptide sequences. The mature APD proteins ranged in size from 6.0 to 37.0 kD, and their pI ranged from 6.2 to 10.8. Intriguingly, five small APD proteins, including APD-2 from A. cerana, and APD 1-3 and APD-like from A. mellifera, were rich in hydrophobic amino acid residues (52%-67%)  with Ala being the most abundant (25%-34%). However, the other five larger APDs, including APD 1-3 from Nasonia vitripennis, and APD-1-like and APD-3-like from A. mellifera, were Gly-rich (21%-30%) proteins with hydrophobic amino acid residues constituting 35%-41% of their amino acid sequences. Multiple alignment and phylogenetic analysis revealed that the APD protein family could be classified into two subfamilies. Subfamily Ⅰ, which contains four low complexity sequences (APD 1-3 from A. mellifera and APD-2 from A. cerana), was characterized by a 33-aa N-terminal conserved motif. The other six larger APD proteins were classified as subfamilyⅡ, which was featured by a 50-aa N-terminal motif and a 16-aa C-terminal motif. The diagnostic features of APD protein family described here will be very helpful for identifying novel apd genes from arthropods.

Key words: Apis mellifera, Apis cerana, cuticular protein, apidermin gene, APD family, sequence analysis, bioinformatical analysis