MICROFIBRIL-ASSOCIATED GLYCOPROTEIN-1 (MAGP-1) IS SPECIFICALLY LOCATED ON THE BEADS OF THE BEADED-FILAMENT STRUCTURE FOR FIBRILLIN-CONTAINING MICROFIBRILS AS VISUALIZED BY THE ROTARY SHADOWING TECHNIQUE

This study used immunoelectron microscopic techniques to define the ul trastructural location of MAGP-1 on the fibrillin-containing microfibr ils of the ocular zonule. A specific anti-MAGP-1 monoclonal antibody ( MAb), 11B, was produced that did not crossreact with fibrillin-1 or ot her microfibrillar proteins. MAb 11B was shown by immunofluorescence t o localize intensely to zonular tissue. Postembedding immunoelectron m icroscopy showed that MAGP-1 was associated with microfibrils througho ut the zonule, with the exception of a narrow band of microfibrils at the junction with the lens capsule. With preembedding labeling, the an ti-MAGP-1 MAb was found to localize in a crossbanding pattern, at inte rvals of about 50 nm, to microfibrils throughout the zonule and along bundles of microfibrils in surrounding vitreous tissue. Rotary shadowi ng of isolated microfibrils showed a ''beads on a string'' morphology with a periodicity of about 50 nm. With immunogold labeling, the anti- MAGP-1 antibody specifically localized on the beads in a symmetrical m anner. Occasionally two gold particles were attached to the same bead, suggesting that multiple MAGP-1 molecules were present in the structu re. The results indicate that MAGP-1 is intimately and regularly assoc iated with the bead regions of fibrillin-containing microfibrils. The findings are consistent with a major structural role for MAGP-1 in mic rofibril biology.