An Intracellular Fragment of Osteoactivin Formed by Ectodomain Shedding Trans located to the Nucleoplasm and Bound to RNA Binding Proteins

Osteoactivin is a type I transmembrane protein upregulated by unloading stresses, including denervation, prolonged bed rest, and space flight, but the regulatory mechanisms of its expression and activation under these conditions remain undefined. Here we report that osteoactivin protein exists in two forms: an intact transmembrane form and a secreted form. The secreted form, the extracellular fragment of osteoactivin, was produced by ectodomain shedding and was released into a culture medium. Amino acid sequence analysis of the carboxy-terminal fragment of osteoactivin (OA-CTF) revealed that cleavage of osteoactivin by proteases occurred both at the cell surface and within the cell membrane. Localization analysis demonstrated translocalization of OA-CTF to the nucleus and the endoplasmic reticulum. Moreover, RNA binding proteins, which regulate pre-mRNA splicing, were identified as OA-CTF binding proteins. These results suggest that OA-CTF formed by ectodomain shedding is involved in the regulation of pre-mRNA splicing.