Structure and biosynthesis of cytoplasmic and secreted variants of gelsolin

Gelsolin is an actin-fragmenting cytoplasmic protein. A functionally similar protein has also been identified in plasma. We have compared the structure of the cytoplasmic and plasma forms of gelsolin and examined their biosynthetic relationships. Plasma gelsolin is larger than cytoplasmic gelsolin (M(r) 93,000 versus 90,000, respectively) and is more positively charged. Partial amino acid sequencing analyses show that the two gelsolins share a common 29 amino acid sequence which lies at the NH₂-terminal and of cytoplasmic gelsolin and spans residues 26-55 of plasma gelsolin. Compared with cytoplasmic gelsolin, plasma gelsolin contains an additional peptide of 25 amino acids at its NH₂ terminus. The human hepatoma-derived cell line, HepG2, synthesizes both the 90-kDa and the 93-kDa gelsolins but secretes only the 93-kDa form. Pulse-chase experiments demonstrate that the rate of disappearance of the 93-kDa gelsolin from the cells corresponds with the rate of appearance of the 93-kDa gelsolin in the medium, whereas the rate of disappearance of the 90-kDa gelsolin is independent of and slower than that of the secreted plasma protein. We conclude that cytoplasmic and plasma gelsolins are structurally similar but not identical, that after synthesis these proteins are processed independently, and that the fate of each is distinct.