A Functional NSP4 Enterotoxin Peptide Secreted from Rotavirus-Infected Cells

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American Society for Microbiology

RESUMO

Previous studies have shown that the nonstructural glycoprotein NSP4 plays a role in rotavirus pathogenesis by functioning as an enterotoxin. One prediction of the mechanism of action of this enterotoxin was that it is secreted from virus-infected cells. In this study, the media of cultured (i) insect cells infected with a recombinant baculovirus expressing NSP4, (ii) monkey kidney (MA104) cells infected with the simian (SA11) or porcine attenuated (OSU-a) rotavirus, and (iii) human intestinal (HT29) cells infected with SA11 were examined to determine if NSP4 was detectable. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis–Western blotting, immunoprecipitation and N-terminal amino acid sequencing identified, in the early media from virus-infected cells, a secreted, cleavage product of NSP4 with an apparent molecular weight of 7,000 that represented amino acids 112 to 175 (NSP4 aa112–175). The secretion of NSP4 aa112–175 was not affected by treatment of cells with brefeldin A but was abolished by treatment with nocodazole and cytochalasin D, indicating that secretion of this protein occurs via a nonclassical, Golgi apparatus-independent mechanism that utilizes the microtubule and actin microfilament network. A partial gene fragment coding for NSP4 aa112–175 was cloned and expressed using the baculovirus-insect cell system. Purified NSP4 aa112–175 increased intracellular calcium mobilization in intestinal cells when added exogenously, and in insect cells when expressed endogenously, similarly to full-length NSP4. NSP4 aa112–175 caused diarrhea in neonatal mice, as did full-length NSP4. These results indicate that NSP4 aa112–175 is a functional NSP4 enterotoxin peptide secreted from rotavirus-infected cells.

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