Regulation of NaCl-induced proline accumulation by calmodulin via modification of proline dehydrogenase activity in Ulva fasciata (Chlorophyta)

Abstract

This paper examined the role of proline dehydrogenase (PDH; EC 1.4.3.1) in salinity-induced (adjusted by increasing NaCl contents in seawater) proline accumulation in the marine chlorophyte Ulva fasciata Delile, and also determined whether calmodulin modulates proline accumulation via a change in the activity and kinetic property of PDH. Upon exposure to varying salinity (30–120‰, that is, 317.9–1858 mМ NaCl in seawater) for 9 h, proline levels increased with increasing salinity, and were highly correlated with salinity (r² = 0.97). In contrast, PDH-specific activity decreased with increasing salinity, and was inversely related with external salinity (r² = 0.95) and proline levels (r² = 0.80). When exposed to 60‰ salinity (831.3 mМ NaCl), PDH-specific activity decreased at hour 2, and proline levels increased at hour 3, as compared to the 35‰ (403.5 mМ NaCl) control. The addition of chlorpromazine (CP, 0.8 µМ) or trifluoperazine (TFP, 1 µМ), both calmodulin binding inhibitors, in seawater enhanced the increase in the levels of proline, but did not affect its K_m value. It also enhanced the decrease in the specific activity and V_max value of PDH in the 60‰-treated thallus discs. Overall, the reduction in catabolism via a decrease in PDH activity is associated with the NaCl-induced proline accumulation in U. fasciata, and the calmodulin-mediated signal transduction might be negatively involved in the regulation of proline accumulation via a modification in PDH properties.