THE ROLE OF CALCIUM IN THE ORGANIZATION OF FIBRILLIN MICROFIBRILS

The microfibrillar glycoprotein fibrillin has a multidomain structure which contains forty-three epidermal growth factor-like motifs with calcium-binding consensus sequences. We have utilized intact microfibrils isolated from human dermal fibroblast cultures to investigate the putative influence of bound calcium on microfibrillar organization and integrity. Incubation with EDTA or EGTA rapidly resulted in gross disruption of microfibril morphology. The treatment induced disorganization of the interbead domains although the regular beaded arrangement was always apparent. These changes were readily reversible on replacing calcium, indicating that the treatment had not compromised microfibrillar integrity. The data localize calcium binding EGF-like repeats to the interbead domains and indicate that lateral packing of fibrillin monomers is calcium-dependent. This arrangement suggests how mutations in epidermal growth factor-like domains of fibrillin might cause the disruption in microfibril organization and interactions which underlies the clinical symptoms of some Marfan syndrome patients.