The rapidly phosphorylated chromosomal 42-kDa protein is a subunit of larger protein complexes

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 novel nuclear 42-kDa casein kinase identified in Chironomus tentans

We have purified and characterised an apparently novel nuclear 42-kDa casein kinase from epithelial cells of Chironomus tentans which comigrates with a phosphoprotein associated with transcriptionally active salivary gland genes. The protein kinase promotes phosphorylation of casein and phosvitin, using either ATP or GTP as phosphate donors, and undergoes autophosphorylation. The casein kinase activity of the 42-kDa protein is sensitive to heparin, 5,6-dichloro-1-beta-D-ribofuranosylbezimidazole (DRB), spermine and spermidine indicating that it is a novel enzyme with similar but not identical properties to casein kinase II or nuclear protein kinase NII.

Analysis of the structural relationships between the DNA-binding phosphoproteins pp42, pp43 and pp44 by in situ peptide mapping

A structural homology is established between three DNA-binding phosphoproteins located in the 42 to 44 kDa range, referred to as pp42, pp43 and pp44, from Chironomus tentans salivary gland cells by in situ peptide mapping. The staining patterns of pp42, pp43 and pp44 which resulted from digestion with Staphylococcus aureus V8, trypsin or papain proteases show the presence of 8 to 15 spots majority of which have identical mobility. In the patterns of the digests generated by treatments with trypsin about 10 spots appear in common between any pair of the protein substrates. In addition, each pattern includes two to three peptides of mobility not present in the other. Thus the peptide mapping of pp42, pp43 and pp44 based on the staining patterns of proteolytic digests suggest the existence of structural homology between the three unlabelled substrates. The proteolytic peptides carrying the rapidly turning over phosphate groups form markedly different electrophoretic patterns than the unlabelled peptides visualized by staining. Treatment of 32P-labelled pp42, pp43 and pp44 with V8 generates only one labelled fragment in the 30 kD range. The cleavage patterns of pp44 produced by chymotrypsin or papain contain seven to ten labelled fragments while those of pp42 and pp43 contain only two. The 32P-labelled tryptic peptides of pp42, pp43 and pp44 exhibit a ladder pattern for each substrate which probably arise by a consecutive removal of 25 to 35 amino acid residues from the primary digestion products pp29, pp29.5 and pp30 by cleavage of four to five putative interdomain regions. The possibility that these three structurally related phosphoproteins belong to the category of transcription factors is discussed.