A GENE ENCODING A VICILIN-LIKE PROTEIN IS SPECIFICALLY EXPRESSED IN FERN SPORES - EVOLUTIONARY PATHWAY OF SEED STORAGE GLOBULINS

The isolation and characterisation of a cDNA coding for a vicilin-like protein of the fern Matteuccia struthiopteris is described. The corre sponding gene is specifically expressed during late stages of spore de velopment. Extensive sequence comparisons suggest that the fern protei n can be considered as a molecular missing link between single-domain germin/spherulin-like proteins and two-domain seed storage globulins o f gymnosperms and angiosperms. Further, evidence is provided for the e xistence of a super-family of structurally related, functionally diffe rent proteins which includes storage globulins of the vicilin and legu min families, a membrane-associated sucrose-binding protein of soybean , a Forssman antigen-binding lectin of velvet bean, the precursor of t he vacuolar membrane bound proteins MP27/MP32 of pumpkin, the embryoge nesis-specific protein Gea8 of carrot, the fern-spore-specific protein described here as well as the functionally diverse family of germins/ germin-like proteins and the spherulins of myxomycetes. We propose tha t seed storage globulins of spermatophytes evolved from desiccation-re lated single-domain proteins of prokaryotes via a duplicated two-domai n ancestor that is best represented by the extant fern spore-specific vicilin-like protein.