Identification of an E3 ubiquitin-protein ligase in the softshell clam (Mya arenaria)

Softshell clams (Mya arenaria) were exposed to dioxin in controlled laborat ory experiments in order to study their molecular response to dioxin exposu re. A complementary DNA (cDNA) fragment with sequence similarity to E3 ubiq uitin-protein ligase appeared to be upregulated in dioxin-exposed clams com pared to controls. E3 covalently ligates ubiquitin onto a protein, targetin g it for degradation. Our findings suggest that the ubiquitin-mediated prot eolytic pathway in the softshell clam may be activated by dioxin exposure. Because the clam E3-predicted amino acid sequence is most similar to a spec ific vertebrate E3 protein (E6-AP), we hypothesize that dioxin may stimulat e ubiquitin-mediated degradation of cell-cycle regulatory proteins, such as the tumor suppressor p53, which promotes cell proliferation. This pathway has been observed in human cervical cancer, Partial cDNA sequence of the cl am E3 has been identified using the differential display polymerase chain r eaction (ddPCR) and RACE (Rapid Amplification of cDNA Ends) PCR; the full-l ength sequence is currently being determined. Discovering the molecular mec hanism(s) stimulated by dioxin exposure in this invertebrate model may cont ribute to a better understanding of the effects of dioxin on marine organis ms. (C) 2000 Elsevier Science Ltd. All rights reserved.