Myocilin, a Component of a Membrane-Associated Protein Complex Driven by a Homologous Q-SNARE Domain

Myocilin is a widely expressed protein with no known function; however, mutations in myocilin appear to manifest uniquely as ocular hypertension and the blinding disease of glaucoma. Using the protein homology/analogy recognition engine (Phyre), we find that the olfactomedin domain of myocilin is similar in sequence motif and structure to a six blade, kelch repeat motif based on the known crystal structures of such proteins Additionally, using sequence analysis, we identify a coiled coil segment of myocilin with homology to human Q-SNARE proteins (inset). Using COS-7 cells expressing full-length human myocilin and a version lacking the C-terminal olfactomedin domain, we identified a membrane associated protein complex containing myocilin by hydrodynamic analysis The myocilin construct that included the coiled coil but lacked the olfactomedin domain formed complexes similar to the full-length protein, indicating that the coded coil domain of myocilin is sufficient for myocilin binding to the large detergent resistant complex. In human retina and retinal pigment epithelium, which express myocilin, we detected the protein in a large, sodium dodecyl Sulfate-resistant, membrane-associated complex. We characterized myocilin in human tissues as either a 15 S complex with an M-r of 405000-440000 yielding a slightly elongated globular shape similar to that of known SNARE complexes or a 6.4 S dimer with an M-r of 108000. By identifying the Q-SNARE homology within the second coil of myocilin and documenting its participation in a SNARE-like complex, we provide evidence of a SNARE domain-containing protein associated with a human disease.