The in vitro phosphorylation of the native rat incisor dentin phosphophoryns

Phosphophoryns are the major non-collagenous proteins of the mineralized matrix of rat incisor dentin. Nearly half the phosphophoryn residues are serines, and 85-90% of these are phosphorylated. Since phosphorylation may be important for phosphophoryn function, it was of interest to identify the kinase(s) responsible for catalyzing their phosphophorylation. Rat osteosarcoma (ROS) 17/2.8 osteoblast-like cells were selected as the enzyme source. Native rat incisor phosphophoryns (RIPP-I, II, III) were not substrates for any of the ROS 17/2.8 messenger-dependent kinases but were phosphorylated by membrane-associated endogenous messenger-independent kinases. These were resolved chromatographically and identified as casein kinase (CK) I and II by elution properties and immunoblotting with a CKII antibody. The CKI preferentially used RIPP-III as substrate, while CKII preferred RIPP-I and II. Heparin at 100 and 500 ng/assay and NaCl at 0.25-0.4 M inhibited phosphorylation of the RIPP by CKI and CKII in parallel. At 10 mM spermine, phosphorylation of RIPP-I and II by CKII, and of RIPP-III by CKI were inhibited, but phosphorylation of RIPP-III by CKII was enhanced. Purified sea star oocyte CKII demonstrated the same substrate specificity and spermine concentration shift as the ROS 17/2.8 CKII. These data show that osteoblast-like cells are a rich source of membrane-bound CKI and CKII activity. The different patterns of phosphorylation of RIPP-I, II, and III further show that they are distinct synthetic products of the odontoblast.