Protein kinases of the chick oviduct: a study of the cytoplasmic and nuclear enzymes

Abstract

Subcellular fractionation of oviduct tissue from estrogen-treated chicks indicated that the bulk of the protein kinase activity of this tissue is located in the cytoplasmic and nuclear fractions, DEAE-cellulose chromatography of cytosol revealed a major peak of cAMP stimulatable activity eluting at 0.2 M KCl. This peak was further characterized and found to exhibit properties consistent with cytoplasmic cAMP dependent protein kinases isolated from other tissues; it had a Km for ATP of 2 X 10(-5) M, preferred basic proteins such as histones, as substrate, and had a M of 165 000. Addition of 10(-6) M cAMP caused the holoenzyme to dissociate into cAMP binding regulatory subunit and a protein kinase catalytic subunit. Extraction of purified oviduct nuclei with 0.3 M KCl released greater than 80% of the kinase activity in this fraction. Upon elution from phospho-cellulose, the nuclear extract was resolved into two equal peaks of kinase activity (designated I and II). Peak I had a sedimentation coefficient of 3S and a Km for ATP of 13 muM. while peak II had a sedimentation coefficient of 6S and a Km for ATP of 9 muM. Both enzymes preferred alpha-casein as a substrate over phosvitin or whole histone, although they exhibited different salt-activity profiles. The cytoplasmic and nuclear enzymes were well separated on phospho-cellulose and this resin was used to quantitate the amount of cAMP dependent histone kinase activity in the nucleus and the amount of casein kinase activity in the cytosol. Protein kinase activity in nuclei from estrogen-stimulated chicks was found to be 40% greater than hormone-withdrawn animals. This increase in activity was not due to translocation of the cytoplasmic protein kinase in response to hormone, but to an increase in nuclear (casein) kinase activity. During the course of this work, we observed small but significant amounts of cAMP binding activity very tightly bound to the nuclear fraction. Solubilization of the binding activity by sonication in high salt allowed comparison studies to be performed which indicated that the nuclear binding protein is identical with the cytoplasmic cAMP binding regulatory subunit. The possible role of the nuclear binding activity is discussed.