Expressions of the Protein Phosphatases PP1 and PP4 during the Embryonic Diapause Process of the Silkworm Bombyx mori.

Abstract

Reversible protein phosphorylation is accomplished by the opposing activities of kinases and phosphatases. We previously demonstrated the regulation of serine/threonine protein phosphatase (PP) type 2A (PP2A) and 2B (PP2B or calcineurin) during the embryonic diapause process of Bombyx mori. In the present study, we further examine the expressions of other PPs (PP1 and PP4) during embryonic stages. An immunoblot analysis showed that Bombyx eggs contained a 38-kDa PP1 catalytic subunit (PP1-C), a 38-kDa PP4 catalytic subunit (PP4-C), and a 120-kDa PP1 nuclear targeting subunit (PNUTS), each of which underwent differential changes between diapause and developing eggs during the embryonic process. In non-diapause eggs, eggs whose diapause initiation was prevented by HCl, and eggs in which diapause had been terminated by chilling diapausing eggs at 5°C for 70 days and then were transferred to 25°C, protein levels of PP1-C and PP4-C remained relatively high during the early embryonic stages and then decreased during middle (for PP1-C) or later (for PP4-C) embryonic stages. However, protein levels of PP1-C and PP4-C in diapause eggs remained at high levels during the first 8 days after oviposition. PNUTS protein levels showed inverse temporal changes, with increased levels being detected during the later embryonic stages of developing eggs. The direct determination of PP1 enzymatic activity showed higher activity in developing eggs than in diapause eggs. Examination of temporal changes in mRNA expression levels of PP1-C and PP4-C showed no difference between HCl-treated and diapause eggs. These results indicated that differential protein levels of PP1-C/PNUTS and PP4-C, and increased enzymatic activity of PP1 were likely related to the embryonic development of B. mori.