Fib420: a normal human variant of fibrinogen with two extended alpha chains

Molecular analysis of scabrous mutant alleles from Drosophila melanogaster indicates a secreted protein with two functional domains

Mutations at the scabrous locus (sca) affect cell-cell signaling during neural development. Twenty-one mutant alleles of scabrous have been analyzed. Many synthesize no sca protein. In others, a defective protein is arrested intracellularly. Two mutants in which protein is not arrested must affect sca protein function outside the cell. Both affect the fibrinogen related domain (FReD), a 200-amino acid segment conserved in fibrinogen, tenascins, and other proteins. In fibrinogen, this region is involved in protein interactions and is altered in human mutations affecting blood clotting. In sca(UM2), an invariant Asp residue is replaced by Asn. In sca(MSKF) allele has dominant negative properties, indicating that the truncated amino-terminal portion interferes with the function of so me other gene product. These mutations show that the conserved FReD is essential for wild-type sca function, but suggest that the amino-terminal domain also interacts with other proteins, but other neural mutations were without effect. Models for the role of a two-domain protein in neural development are discussed.

The minor form alpha' chain from lamprey fibrinogen is rapidly crosslinked during clotting

Lampreys have two genes for the alpha chains of fibrinogen, the second of which encodes a minor form with a carboxyl-terminal domain homologous to the carboxyl-terminal domains of beta and gamma chains. Initially, we referred to the alternative chain as alpha-II; we now use the designation alpha' in order to facilitate reference to crosslinked dimers. Antisera raised to synthetic peptides based on the cDNA sequence confirmed that the alpha' chain was present in fibrinogen prepared directly from plasma. The same antibodies were used to determine the size and properties of the carboxyl-terminal domain after its release by mild tryptic digestion, a fragment of apparent molecular weight 35,000-40,000 being produced. Unlike fragment D generated in the same digestions, the alpha' fragment did not bind to Gly-Pro-Arg or Gly-Val-Arg peptide affinity columns. During clotting under conditions where factor XIII is active, the alpha' chains became crosslinked very much more rapidly than ordinary alpha chains, the principal product being an apparent dimer, but smaller amounts of higher multimers being detectable. The crosslinking was inhibited by various amines, as well as by peptides that prevent polymerization.