A novel secreted, cell-surface glycoprotein containing multiple epidermal growth factor-like repeats and one CUB domain is highly expressed in primary osteoblasts and bones

We have previously identified a novel family of secreted, cell-surface proteins expressed in human vascular endothelium. To date, two family members have been described, sharing a characteristic domain structure including an amino-terminal signal peptide followed by multiple copies of epidermal growth factor (EGF)-like repeats, a spacer region, and one CUB domain at the carboxyl terminus. Thus, this family was termed SCUBE for signal peptide-CUB-EGF-like domain containing proteins. Here we described the identification and characterization of one additional member of the SCUBE family named SCUBE3 in humans, sharing an overall 60% protein identity and a similar domain structure with other family members. Real-time quantitative reverse transcriptase-PCR and Northern blot analyses revealed that this gene is highly expressed in primary osteoblasts and the long bones ( humerus and femur), followed by a low level of expression in human umbilical vein endothelial cells and in heart. When overexpressed in human embryonic kidney 293T cells, the recombinant hSCUBE3 protein is a secreted glycoprotein that can form oligomers tethered to the cell surface. Interestingly, the secreted hSCUBE3 protein can be further proteolytically processed by a serum-associated protease to release the EGF-like repeats from the CUB domain. The SCUBE3 gene is mapped to human chromosome 6p21.3, a region that has been linked with the locus for a rare form of metabolic bone disease, Paget's disease of bone 1. Together, this novel secreted, cell-surface osteoblast protein may act locally and/or distantly through a proteolytic mechanism, and may play an important role in bone cell biology.