Involvement of a calcium-dependent protein kinase in hypoosmotic turgor regulation in a brackish water characeae Lamprothamnium succinctum

Calcium ion is a key messenger in turgor regulation of internodal cells of Lamprothamnium succinctum in response to hypoosmotic treatment. An increase in the concentration of cytosolic free calcium ion ([Ca2+](c)) is prerequisite for the turgor regulation [Okazaki and Tazawa (1990) J. Membr. Biol. 114: 189]. We examined whether or not a calcium-dependent protein kinase (CDPK) is involved in the Ca2+-mediated turgor regulation of Lamprothamnium cells. A 53-kDa CDPK which phosphorylated preferentially histone H1 but poorly myelin basic protein or casein, was detected in the cell extract of Lamprothamnium by an in-gel protein kinase assay. This protein kinase was detected by Western blotting and was immunoprecipitated using an anti-Dunaliella tertiolecta CDPK antibody which can neutralize the Dunaliella CDPK activity [Yuasa et al. (1995) Plant Cell Physiol. 36: 699]. The 53-kDa CDPK was partially purified from Lamprothamnium and its activity was shown to be inhibited by the antibody and K-252a, a protein kinase inhibitor. Microinjection of the antibody into the cytosol of Lamprothamnium cells inhibited the decrease in turgor pressure in response to hypoosmotic treatment. However, a transient increase in [Ca2+](c), which was suggested by a transient reduction of the velocity of cytoplasmic streaming, was induced in antibody-injected cells after hypoosmotic treatment. Turgor regulation upon hypoosmotic treatment was inhibited when the cells were treated with K-252a. These results imply that CDPK of Lamprothamnium functions at a downstream position of Ca2+-mobilization in processing turgor regulation in response to hypoosmotic treatment.