Truncated profibrillin of a Marfan patient is of apparent similar size as fibrillin: intracellular retention leads to over-N-glycosylation.

We studied profibrillin-1 (proFib) synthesis and microfibril formation in cultured fibroblasts from an individual with severe Marfan syndrome harboring a premature stop codon (W2756ter) in one FBN1 allele. Rotary shadowing analysis of extracellular matrix produced by these cells revealed the presence of only a very few intact microfibrils which showed marked disorganisation within the interbeaded domains. Metabolic pulse-chase studies identified intracellularly a population of truncated proFib molecules which were secreted more slowly than the normal proFib derived from the normal allele. Culture media contained strikingly reduced amounts of wild-type proFib in comparison to fibrillin (Fib). Our findings imply that (1) the truncated proFib is secreted and disturbs microfibril assembly; (2) the mutation is probably close to a putative cleavage site in the proFib C terminus necessary for the conversion of proFib to Fib; (3) the truncated proFib is over-N-glycosylated due to intracellular retention rather than incomplete cleavage of proFib with persistence of N-glycosylated sites; (4) not all potential N-glycosylation sites in proFib seem to be normally used, since we could produce over-N-glycosylated proFib in normal cells by brefeldin A mediated intracellular captivation and subsequent appearance of over-glycosylated Fib in culture medium upon removal of the compound. It is conceivable that post-translational over-modification might be important for modulating the phenotype of FBN1 mutations in Marfan syndrome.