LOCALIZATION OF BONE SIALOPROTEIN (BSP) TO GOLGI AND POST-GOLGI SECRETORY STRUCTURES IN OSTEOBLASTS AND TO DISCRETE SITES IN EARLY BONE-MATRIX

Bone sialoprotein (BSP), a bone matrix-enriched glycoprotein containing the Arg-Gly-Asp (RGD) motif and endowed with cell binding properties, was localized in osteoblasts and early bone matrix of developing rat bone at the ultrastructural level. Preliminary light microscopic observations indicated that intracellular labelling was restricted to a paranuclear dot corresponding to the ''negative Golgi image'' of classical histology. The same pattern was observed whether antisera against the fully glycosylated protein or a peptide antiserum to a stretch of amino acids in human BSP sequence were employed. At the EM level, we obtained labeling over the Golgi area of osteoblasts but not over the rER. The labeling was concentrated over distensions of the trans Golgi and over pro-secretory granules. In the matrix, BSP was distributed in a non-random manner. The label was concentrated over spherical aggregates of finely fibrillar material corresponding to the sites of early mineral deposition (so-called ''mineralization nodules''). Such BSP-positive foci were seen both close to and away from the cell surface. The predominant association of BSP with Golgi and post-Golgi secretory structures and its absence from rER, as well as the reproducibility of the same pattern of localization with different antisera, might indicate a slow transit of the protein through the Golgi, not necessarily associated with protein glycosylation.