Identification of a ubiquitin-like protein in the mammalian vitreous humor

An 8 kDa ubiquitin-like peptide (ULP) was isolated by high performance liquid chromatography from the rabbit vitreous humor, and the N-terminal amino acid sequence of this peptide showed complete homology with ubiquitin. Western blot revealed the presence of free ULP in both the iris-ciliary (IC) complex and the aqueous humor extracts. In the IC complex, fluorescence and immunoelectron microscopy detected high concentrations of ULP in the posterior epithelial cells, suggesting this tissue as a possible source of ULP in the ocular fluids. Significantly, this is the first time that the presence of free ULP has been reported in mammalian extracellular fluids. Furthermore, we recently demonstrated that the 8 kDa fraction of vitreous humor containing ULP is a potent inhibitor of protein synthesis [Banerjee et al. (1992): J Cell Biochem 49:66-73]. These findings taken together suggest a novel biological role for ULP in the control of lens cell growth.