A novel nuclear 42-kDa casein kinase identified in Chironomus tentans

Phosphorylation dependence of the initiation of productive transcription of Balbiani ring 2 genes in living cells

Using polytene chromosomes of salivary gland cells of Chironomus tentans, phosphorylation state-sensitive antibodies and the transcription and protein kinase inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), we have visualized the chromosomal distribution of RNA polymerase II (pol II) with hypophosphorylated (pol IIA) and hyperphosphorylated (pol II0) carboxyl-terminal repeat domain (CTD). DRB blocks labeling of the CTD with 32Pi within minutes of its addition, and nuclear pol II0 is gradually converted to IIA; this conversion parallels the reduction in transcription of protein-coding genes. DRB also alters the chromosomal distribution of II0: there is a time-dependent clearance from chromosomes of phosphoCTD (PCTD) after addition of DRB, which coincides in time with the completion and release of preinitiated transcripts. Furthermore, the staining of smaller transcription units is abolished before that of larger ones. The staining pattern of chromosomes with anti-CTD antibodies is not detectably influenced by the DRB treatment, indicating that hypophosphorylated pol IIA is unaffected by the transcription inhibitor. Microinjection of synthetic heptapeptide repeats, anti-CTD and anti-PCTD antibodies into salivary gland nuclei hampered the transcription of BR2 genes, indicating the requirement for CTD and PCTD in transcription in living cells. The results demonstrate that in vivo the protein kinase effector DRB shows parallel effects on an early step in gene transcription and the process of pol II hyperphosphorylation. Our observations are consistent with the proposal that the initiation of productive RNA synthesis is CTD-phosphorylation dependent and also with the idea that the gradual dephosphorylation of transcribing pol II0 is coupled to the completion of nascent pol II gene transcripts.

Induced release of cell surface protein kinase yields CK1- and CK2-like enzymes in tandem

Several types of cell exhibit cell surface protein kinase (ecto-PK) activities with Ser/Thr-specificity. Ecto-PK sharing certain characteristics of protein kinase CK2 can be detached from intact cells by interaction with exogenous substrates (Kübler, D., Pyerin, W., Burow, E., and Kinzel, V. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 4021-4025). However, a detailed molecular analysis of this ecto-PK was hampered by the vanishingly small amounts of labile enzyme protein obtained by substrate-inducible enzyme release. We now describe the stabilization and enrichment of released ecto-PK by precipitation with polyethylene glycol followed by affinity chromatography on heparin-agarose. Ecto-PK is shown to consist of two separate forms released in tandem, ecto-PK I and ecto-PK II. Comparison with cell homogenates as well as cell surface biotinylation experiments excluded contamination with intracellular PK. Purified ecto-PK I and ecto-PK II exhibit respectively selective phosphorylation of CK1- and CK2-specific peptide substrates, a complementary sensitivity to inhibitory agents and a differential use of the cosubstrates ATP and GTP. Ecto-PK I consists of a 40-kDa moiety; the ecto-PK II is an ensemble of three components of 43- and 40-kDa (catalytic subunits) and a noncatalytic 28-kDa subunit. In addition, components of the ecto-PK II react with CK2-specific antibodies. Further, comparative peptide mapping and the results of mass spectrometry in combination with assignment of amino acid sequences confirmed that ecto-PK II is closely related if not identical to the protein kinase CK2. Assays with intact cells that result in the phosphorylation of a variety of endogenous membrane proteins showed that both ecto-PKs participate, and further, certain ecto-PK substrates become preferentially labeled by one or another of the enzymes, whereas others are phosphorylated by both ecto-PK activities.

The salivary gland 42-kDa phosphoprotein is a single-stranded DNA-binding protein with characteristics of the epithelial casein kinase N42 in Chironomus tentans

The DNA-binding and phosphorylation properties of a rapidly phosphorylated nuclear 42-kDa phosphoprotein and of its two structurally related proteins, pp43 and pp44 in Chironomus tentans salivary glands were investigated. pp42, pp43 and pp44 bind promoter probes of the ecdysterone controlled I-18C gene and of the joint histone H2A/H2B genes in a sequence-selective and single-stranded DNA (ssDNA) specific manner. Rapid phosphorylation appears to give pp42 and pp43 uniquely hydrophilic characters making them soluble in the aqueous phase during phenol treatment. Dephosphorylation of the nuclear proteins markedly stimulates the ssDNA-binding activity of pp42 but not of pp43 and pp44. All three phosphoproteins are sensitive to heparin and the transcription inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) in vitro, but their sensitivity to heparin is more than one order of magnitude lower than that of casein kinase II. The heparin sensitivity of pp42 and pp43 is, however, similar to that described for a previously identified nuclear 42-kDa phosphoprotein in a Chironomus tentans epithelial cell line, casein kinase N42 (CKN42). pp42 and pp43 bind with high affinity to a Phosvitin-Sepharose matrix, like casein kinase I, II and N42, and can be eluted with high salt buffers from the affinity column. In intact salivary gland cells, microinjected (gamma-32P)GTP labels pp42 in a heparin sensitive manner, and this GTP-phosphorylation of pp42 could be competed out by a large excess of phosvitin. (gamma-32P)ATP-based phosphorylation of pp42 was uninfluenced by phosvitin in intact cells. The experimental data suggest that the salivary gland 42-kDa phosphoprotein, pp42, is a ssDNA-binding protein with characteristics of the epithelial CKN42.

A majority of casein kinase II alpha subunit is tightly bound to intranuclear components but not to the beta subunit

Nuclear casein kinase II (CK II) was purified from an epithelial cell line of Chironomus tentans and characterized. The intracellular distribution of CK II and its two intracellular subunits (alpha and beta) was analysed by immunoblotting. The apparent molecular weights of the alpha and beta subunits were estimated to be 36 and 28 kDa, respectively. Like other purified CK II preparations, CK II from Chironomus tentans is able to use ATP or GTP for phosphorylation of casein and phosvitin, and its activity is strongly inhibited by heparin and by the transcription inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). Due to their differential solubilities in NaCl and (NH4)2SO4 solutions, individual alpha and beta subunit pools could be detected. More than 85% of the total immunostainable alpha subunit and essentially all immunoreactive individual beta subunit and heterooligomeric enzyme molecules were localised to the nucleus. Unexpectedly, more than 80% of this nuclear alpha subunit was insoluble in 0.35 M NaCl, while all individual beta subunit and heterooligomeric enzyme molecules were solubilized under the same conditions. Of the 0.35 M NaCl soluble kinase fractions, the active multisubunit form of CK II precipitated in 50% (NH4)2SO4 and could thus be separated from the free beta subunit, which precipitated at 60% and 80% (NH4)2SO4. These results suggest that a major portion of the nuclear CK II alpha subunit does not form heterooligomeric structures with the beta subunit, but binds tightly to nuclear components.