Four factor VIII light chain constructs containing hemophilia A mutations at R2304
and R2307 were prepared and expressed in mammalian cells. These mutations are located
in a putative phosphatidylserine binding site identified by peptide studies (spanning
amino acids 2303-2332). The levels of all four mutants in conditioned medium were
significantly less than wild type by immunoprecipitation and ELISA. R2304H and wild
type factor VIII light chains were concentrated by cation exchange chromatography
from medium. R2304H and wild type factor VIII light chains bound immobilized phosphatidylserine
similarly. The reconstituted cofactor activity of R2304H factor VIII light chain was
slightly greater than wild type factor VIII light chain. These results are consistent
with the recently reported crystal structure of factor VIII C2 domain that suggests
R2304H is not directly involved in phospholipid binding. The observed clinical phenotype
is probably due to decreased circulating levels of a functional protein.
Keywords
Factor VIII - factor VIII light chain - hemophilia - phospholipid binding