Sea cucumber (Apostichopus japonicus) was one of the important aquaculture species in the north coast of China. The studies on function and mechanism of immune-related genes can provide a reference to the prevention of diseases in the cultured A. japonicus. In this study，a full-length cDNA sequence of profilin gene from A. japonicus was obtained by constructing cDNA libraries and sequencing analysis, which was 787 bp and includes a 205 bp 5′- untranslated region (UTR), 378 bp ORF encoding 125 amino acids with a conserved PROF domain, and a 204 bp 3′-UTR; and the predicted molecular weight was 13.4 kDa. The conserved PROF domain included acting interaction sites, poly-L-proline binding sites, and PIP2-interaction sites. The deduced amino acid sequence showed 46% amino acid sequence identity to the Profilin of Trichoplax adhaerens and Saccoglossus kowalevskii. The profilin mRNA levels were detected by using quantitative real-time PCR at different developmental stages of sea cucumber, including unfertilized egg, fertilized egg, cellulous stages, blastula, gastrula, early auricularia, auricularia, late auricularia, doliolaria, pentactula and juvenile. The expression patterns were also detected in different tissues of young sea cucumbers including intestines, coelomocytes, respiratory trees and body walls. The profilin expression levels were low from unfertilized eggs to gastrula, however, an increase in transcript levels was observed from early auricularia to juvenile. The highest expression level was found in the coelomocytes of young sea cucumbers. LPS was a potent activator of the echinoderm immune system，and after LPS challenge, the highest expression of profilin mRNA was observed in the coelomocytes.
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