DEVELOPMENTAL ASSOCIATION OF THE BETA-GALACTOSIDE-BINDING PROTEIN GALECTIN-1 WITH THE NUCLEAR MATRIX OF RAT CALVARIAL OSTEOBLASTS

The protein composition of the nuclear matrix changes significantly as the osteoblast matures from a proliferating pre-osteoblast to an oste ocyte embedded in a mineralized matrix. These matrix protein are the r esult of developmental stage-specific gene expression during osteoblas t differentiation. To isolate nuclear matrix proteins unique to the bo ne phenotype we analyzed nuclear matrix preparations from cultures of rat calvarial osteoblasts by high resolution two-dimensional gel elect rophoresis at two different stages: proliferation (day 3) and differen tiation (day 18, mineralized). We characterized one protein (14 kDa; p i 5.0), that was detectable only in the nuclear matrix of differentiat ed osteoblasts, By mass spectrometry and microsequencing, this protein was identified as the beta-galactoside-binding protein galectin-1. Bo th immunofluorescence staining of nuclear matrix preparations with the galectin-1 antibody and western blot analysis of subcellular fraction s 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 differ entiation. Galectin-1 is present in the cytoplasmic and nuclear fracti ons in both proliferating and differentiated osteoblasts. However, its only stable binding is to the nuclear matrix of the differentiated os teoblast; but, in proliferating osteoblasts, galectin-1 is not retaine d in the nuclear matrix. Taken together, our results suggest that deve lopmental association of galectin-1 with the nuclear matrix reflects d ifferential subnuclear binding of galectin-1 during osteoblast differe ntiation.