Dependence on Vitamin K-dependent Protein S for Eukaryotic Cell Secretion of the β-Chain of C4b-binding Protein

The anticoagulant vitamin K-dependent protein S (PS) circulates in plasma in two forms, 30% free and 70% being bound to the complement regulatory protein C4b-binding protein (C4BP). The major C4BP isoform consists of 7 alpha-chains and 1 beta-chain (C4BP beta(+)), the chains being linked by disulfide bridges. PS binds to the beta-chain with high affinity. In plasma, PS is in molar excess over C4BP beta(+) and due to the high affinity, all C4BP beta(+) molecules contain a bound PS. Taken together with the observation that PS-deficient patients have decreased levels of C4BP beta(+), this raises the question of whether PS is important for secretion of the beta-chain from the cell. To test this hypothesis, HEK293 cells were stably and transiently transfected with beta-chain cDNA in combinations with cDNAs for PS and/or the alpha-chain. The concentration of beta-chains in the medium increased after co-transfection with PS cDNA, but not by alpha-chain cDNA, suggesting secretion of the beta-chains from the cells to be dependent on concomitant synthesis of PS, but not of the alpha-chains. Thus, beta-chains that were not disulfide-linked to the alpha-chains were secreted in complex with PS, either as monomers or dimers. Pulse-chase demonstrated that the complexes between PS and beta-chain were formed intracellularly, in the endoplasmic reticulum. In conclusion, our results demonstrate that successful secretion of beta-chains depends on intracellular complex formation with PS, but not on the alpha-chains. This provides an explanation for the decreased beta-chain levels observed in PS-deficient patients.