Most extracellular proteins consist of various modules with distinct functions. 
Mutations in one common type, the calcium-binding epidermal growth factor-like 
module (cbEGF), can lead to a variety of genetic disorders. Here, we describe as 
a model system structural and functional consequences of two typical mutations 
in cbEGF modules of fibrillin-1 (N548I, E1073K), resulting in the Marfan 
syndrome. Large (80-120 kDa) wild-type and mutated polypeptides were 
recombinantly expressed in mammalian cells. Both mutations did not alter 
synthesis and secretion of the polypeptides into the culture medium. Electron 
microscopy after rotary shadowing and comparison of circular dichroism spectra 
exhibited minor structural differences between the wild-type and mutated forms. 
The mutated polypeptides were significantly more susceptible to proteolytic 
degradation by a variety of proteases as compared with their wild-type 
counterparts. Most of the sensitive cleavage sites were mapped close to the 
mutations, indicating local structural changes within the mutated cbEGF modules. 
Other cleavage sites, however, were observed at distances beyond the domain 
containing the mutation, suggesting longer range structural effects within 
tandemly repeated cbEGF modules. We suggest that proteolytic degradation of 
mutated fibrillin-1 may play an important role in the pathogenesis of Marfan 
syndrome and related disorders.