Cloning and sequence determination of rat dentin sialoprotein, a novel dentin protein

Dentin sialoprotein (DSP) is a 53-kDa protein isolated from rat dentin. It contains 29.6% carbohydrate (including 9% sialic acid) and has an overall composition similar to that of the bone sialoproteins osteopontin and bone sialoprotein (i.e. rich in Asp, Ser, Glu, and Gly). Using a monospecific anti-DSP polyclonal antibody to screen a rat incisor odontoblast cDNA library, a cDNA clone was isolated and sequenced. This approximately 750- base pair clone contained a DNA sequence corresponding to the NH₂-terminal 9 amino acids of DSP. A second cDNA clone was isolated by using the first cDNA as a probe to rescreen the library. This second clone had the full-length DSP coding region. From the sequence, we deduced that the DSP cDNA coded for 366 amino acids, predominantly Asp, Ser, Glu, and Gly. The amino acid composition calculated for this sequence was very similar to that for purified DSP reported earlier; likewise the deduced molecular weight (53,045) was essentially identical to that determined by sedimentation equilibrium. Six potential N-linked glycosylation sites were present in the predicted DSP sequence. No Arg-Gly-Asp sequence was found, and the sequence for DSP was dissimilar to those of osteopontin and bone sialoprotein. Multiple transcripts near 4.6 and 1.5 kilobases were detected by Northern blot analysis in the incisor of 21-day-old rat and the tooth germ of the newborn rat. Consistent with previous immunohistochemical findings, no transcripts were detected in brain, salivary gland, heart, muscle, spleen, kidney, intestine, lung, liver, pancreas, tibia, calvaria, or osteoblast-like osteosarcoma (ROS 17/2.8) cells, indicating that DSP is specifically expressed by odontoblasts and related cells.