Functional domains on elastin and microfibril-associated glycoprotein involved in elastic fibre assembly.

Studies in vitro suggest that the C-terminus of tropoelastin mediates elastin polymerization through an interaction with microfibril-associated proteins. In this study we have used cultured auricular chondrocytes as a model system to examine whether this interaction is critical for elastic fibre formation in vivo . Auricular chondrocytes, which deposit an abundant elastic fibre matrix, were cultured in the presence of Fab fragments of antibodies directed against the C-terminus (CT e ) or an N-terminal domain (AT e ) of tropoelastin. Immunofluorescent staining of the extracellular matrix deposited by the cells showed that the CT e antibody inhibited the deposition of elastin without affecting microfibril structure. Cells grown under identical conditions in the presence of AT e , however, formed fibres that stained normally for both elastin and microfibril proteins. Chondrocytes cultured in the presence of microfibril-associated glycoprotein (MAGP):21–35, an antibody directed against a domain near the N-terminus of MAGP, did not organize tropoelastin into fibres. However, immunostaining for MAGP and fibrillin revealed normal microfibrils. In agreement with the immunofluorescence staining patterns, fewer elastin-specific cross-links, indicative of insoluble elastin, were detected in the extracellular matrix of cells cultured in the presence of CT e . The medium from these cultures, however, contained more soluble elastin, consistent with an antibody-induced alteration of elastin assembly but not its synthesis. Northern analysis of antibody-treated and control cultures substantiated equivalent levels of tropoelastin mRNA. These results confirm that the C-terminus of tropoelastin interacts with microfibrils during the assembly of elastic fibres. Further, the results suggest that the interaction between tropoelastin and microfibrils might be mediated by a domain involving the N-terminal half of MAGP.