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.