ALTERNATIVE SPLICING OF REPETITIVE UNITS IS RESPONSIBLE FOR THE POLYDISPERSITIES OF INTEGUMENTARY MUCIN B.1 (FIM-B.1) FROM XENOPUS-LAEVIS

Frog integumentary mucin B.1 (FIM-B.1) represents a polymorphic extrac ellular mosaic protein which contains tandemly arranged serine/threoni ne-rich modules as well as cysteine-rich domains. The latter are proba bly important for oligomerization of FIM-B.1 and have also been found in many proteins of the complement cascade as well as regions homologo us to von Willebrand factor. The repetitive modules are targets for ex tensive O-glycosylation. Previous cDNA cloning experiments clearly est ablished polydispersities within the same individual, which originate from deletions/insertions in the repetitive domain. Here, we analyse p art of the corresponding genomic region. Each repetitive unit as well as the cysteine-rich domain is encoded by an individual class 1-1 exon typical of shuffled modules. Alternative splicing of these multiple c assettes creates the polydisperse FIM-B.1 transcripts.