[¦sī·klik ¦ā¦em¦pē di¦pen·dənt ‚pro‚tēn ′kī‚nās] (biochemistry) A serine/threonine protein kinase that phosphorylates a variety of substrates and regulates many important processes such as cell growth and differentiation and the flow of ions across the cell membrane. Also known as protein kinase A; PKA.