Activation of a 74 kDa plasma membrane protein kinase by hyperosmotic shocks in the halotolerant alga Dunaliella salina

Abstract

The purpose of this study was to find out whether protein kinases are involved in the osmotic response of the halotolerant green alga Dunaliella salina . Hyperosmotic shock leads to a rapid transient activation of two different protein kinases, identified by an in-gel assay: A 40 kD kinase (PK 40), which is activated within seconds after osmotic shocks, and a 74 kD kinase (PK 74), activated a few minutes after the shock. Subcellular fractionation shows that PK 40 is a soluble enzyme whereas PK 74 is a membrane-associated protein localized in a plasma membrane-enriched microsomal fraction. Heat shock and cytoplasmic acidification activate a 40 kD, but not a 74 kD protein kinase. Both PK 40 and PK 74 phosphorylate histone at threonine residues and do not require calcium ions. The following results suggest that PK 74 may be a novel type of a MEK 1 kinase: 1) PK 74 phosphorylates ERK 1, a specific substrate of mammalian MEK 1, and 2). Antibodies against a conserved peptide of MEK 1 cross-react specifically with a 74 kD component localized at the plasma membrane fraction, which possesses the PK 74 activity. It is suggested that PK 74 is activated by osmotically-induced structural changes at the plasma membrane. Its possible role in the osmotic signal transduction is discussed.