The protein composition of the nuclear matrix changes significantly as the osteoblast matures from a proliferating pre-osteoblast to an osteocyte embedded in a mineralized matrix. These matrix protein are the result of developmental stage-specific gene expression during osteoblast differentiation. To isolate nuclear matrix proteins unique to the bone phenotype we analyzed nuclear matrix preparations from cultures of rat calvarial osteoblasts by high resolution two-dimensional gel electrophoresis at two different stages: proliferation (day 3) and differentiation (day 18, mineralized). We characterized one protein (14 kDa; pI 5.0), that was detectable only in the nuclear matrix of differentiated osteoblasts. By mass spectrometry and microsequencing, this protein was identified as the beta -galactoside-binding protein galectin-1. Both immunofluorescence staining of nuclear matrix preparations with the galectin-1 antibody and western blot analysis of subcellular fractions confirmed that galectin-1 is only associated with the nuclear matrix in differentiated osteoblasts as the result of differential retention. Galectin-1 protein and mRNA are present throughout osteoblast differentiation. Galectin-1 is present in the cytoplasmic and nuclear fractions in both proliferating and differentiated osteoblasts. However, its only stable binding is to the nuclear matrix of the differentiated osteoblast; but, in proliferating osteoblasts, galectin-1 is not retained in the nuclear matrix. Taken together, our results suggest that developmental association of galectin-1 with the nuclear matrix reflects differential subnuclear binding of galectin-1 during osteoblast differentiation.
Developmental association of the beta-galactoside-binding protein galectin-1 with the nuclear matrix of rat calvarial osteoblasts
