Identification of a matrix-binding domain in MAGP1 and MAGP2 and intracellular localization of alternative splice forms.

MAGP1 is a small molecular mass protein associated with microfibrils in the extracellular matrix (ECM). To identify the molecular basis of its interaction with other microfibrillar proteins, deletion constructs of MAGP1 were expressed in a mammalian cell system that served as a model for microfibril assembly. This study identified a 54-amino acid sequence in the carboxyl-terminal region of the protein that defines a matrix-binding domain that is sufficient to target MAGP1 to the ECM. Site-directed mutagenesis demonstrated that binding activity is dependent on the presence of 7 cysteine residues in this sequence. MAGP2 contains a sequence similar to the matrix-binding domain of MAGP1, but could not associate with the ECM because of a single amino acid change. Two naturally occurring MAGP1 splice variants, MAGP1B (human-specific) and MAGP1D (found in mice), localized intracellularly when expressed as chimeric proteins with green fluorescent protein in rat lung fibroblasts. This suggests a second action site for MAGP1.