Identification of a S6 kinase transcript in the Chinese mitten crab Eriocheir sinensis and its molting-related expression in muscle tissues

Abstract

Ribosomal protein S6 kinases (S6Ks) are key targets downstream of the mechanistic target of rapamycin (mTOR) signaling pathway that are activated by mTOR complex 1 phosphorylation to promote protein synthesis, and are involved in muscle growth and pathology. In this study, we cloned a 1560-base pair (bp) complementary DNA sequence encoding a unique S6K1, designated EsS6K1, in the Chinese mitten crab Eriocheir sinensis. EsS6K1 contains a 54-bp 5′ untranslated region, a 246-bp 3′ untranslated region and a 1260-bp coding sequence that encodes 419 amino acids. EsS6K1 contains a conserved kinase domain, a TOR signaling motif, and a turn motif; it lacks a C-terminal autoinhibitory domain, and the conventional threonine in the hydrophobic motif is replaced with serine. Real-time quantitative reverse transcription–polymerase chain reaction experiments showed that EsS6K1 expression was highest in the ovaries, followed by the testes and the eyestalks, in mature crabs. In juvenile crabs, the relative messenger RNA levels of EsS6K1 in the claw, walking leg and abdominal muscles were significantly higher in the late premolt (D3-4) stage than in the intermolt (C) stage. After ecdysis, the expression of EsS6K1 in the walking leg and abdominal muscles declined to the level in the C stage, but the expression in claw muscles remained as high as that in the D3-4 stage. The findings showed that EsS6K1 likely plays roles in muscle changes during crustacean molting. These results provide valuable insights into the mechanism underlying crustacean muscle growth induced by molting.