Posttranslational modifications of microfibril associated glycoprotein-1 (MAGP-1).

Microfibril-associated glycoprotein-1 (MAGP-1) is a small molecular weight protein associated with extracellular matrix microfibrils. Biochemical studies have shown that MAGP-1 undergoes several posttranslational modifications that may influence its associations with other microfibrillar components. To identify the sites in the molecule where posttranslational modifications occur, we expressed MAGP-1 constructs containing various point mutations as well as front and back half truncations in CHO cells. Characterization of transiently expressed protein showed that MAGP-1 undergoes O-linked glycosylation and tyrosine sulfation at sites in its amino-terminal half. This region of the protein also served as a major amine acceptor site for transglutaminase and mediated self-assembly into high molecular weight multimers through a glutamine-rich sequence. Fine mapping of the modification sites through mutational analysis demonstrated that Gln20 is a major amine acceptor site for the transglutaminase reaction and confirmed that a canonical tyrosine sulfation consensus sequence is the site of MAGP-1 sulfation. Our results also show that O-glycosylation occurs at more than one site in the molecule.

The microfibrillar proteins MAGP-1 and fibrillin-1 form a ternary complex with the chondroitin sulfate proteoglycan decorin.

MAGP-1 and fibrillin-1, two protein components of extracellular microfibrils, were shown by immunoprecipitation studies to interact with the chondroitin sulfate proteoglycan decorin in the medium of cultured fetal bovine chondrocytes. Decorin interacted with each protein individually and with both proteins together to form a ternary complex. Expression of truncated fibrillin-1 proteins in Chinese hamster ovary cells localized proteoglycan binding to an amino-terminal region near the proline-rich domain. A spatially analogous fibrillin-2 truncated protein did not coprecipitate the same sulfated molecule, suggesting that chondroitin sulfate proteoglycan binding in this region is specific for fibrillin-1. An interaction between fibrillin and MAGP-1 was also observed under culture conditions that abrogated decorin secretion, suggesting that the two microfibrillar proteins can associate in the absence of the proteoglycan. Sulfation of matrix proteins is important for elastic fiber assembly because inhibition of sulfation was shown to prevent microfibrillar protein incorporation into the extracellular matrix of cultured cells.

Interaction of tropoelastin with the amino-terminal domains of fibrillin-1 and fibrillin-2 suggests a role for the fibrillins in elastic fiber assembly.

Alignment of tropoelastin molecules during the process of elastogenesis is thought to require fibrillin-containing microfibrils. In this study, we have demonstrated that amino-terminal domains of two microfibrillar proteins, fibrillin-1 and fibrillin-2, interact with tropoelastin in solid phase binding assays. The tropoelastin-binding site was localized to a region beginning at the glycine-rich and proline-rich regions of fibrillin-2 and fibrillin-1, respectively, and continuing through the second 8-cysteine domain. Characterization of the binding requirements using the fibrillin-2 construct found that a folded, secondary structure was necessary for binding. Furthermore, binding between tropoelastin and fibrillin was mediated by ionic interactions involving the lysine side chains of tropoelastin. The importance of the lysine side chains was corroborated by the finding that the fibrillin-2 construct did not bind to mature elastin, whose lysine side chains have been modified to form cross-links. Interestingly, there was no interaction between the fibrillin constructs and tropoelastin in solution phase, suggesting that binding of tropoelastin to a solid substrate exposes a cryptic binding site. These results suggest that fibrillin plays an important role in elastic fiber assembly by binding tropoelastin and perhaps facilitating side chain alignment for efficient cross-linking.

N-terminal domains of fibrillin 1 and fibrillin 2 direct the formation of homodimers: a possible first step in microfibril assembly.

Aggregation of fibrillin molecules via disulphide bonds is postulated to be an early step in microfibril assembly. By expressing fragments of fibrillin 1 and fibrillin 2 in a mammalian expression system, we found that the N-terminal region of each protein directs the formation of homodimers and that disulphide bonds stabilize this interaction. A large fragment of fibrillin 1 containing much of the region downstream from the N-terminus remained as a monomer when expressed in the same cell system, indicating that this region of the protein lacks dimerization domains. This finding also confirms that the overexpression of fibrillin fragments does not in itself lead to spurious dimer formation. Pulse-chase analysis demonstrated that dimer formation occurred intracellularly, suggesting that the process of fibrillin aggregation is initiated early after biosynthesis of the molecules. These findings also implicate the N-terminal region of fibrillin 1 and fibrillin 2 in directing the formation of a dimer intermediate that aggregates to form the functional microfibril.