Abstract
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 (Mr 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 end 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.
Highlights
Gelsolin is anactin-fragmenting cytoplasmic pro- paper, we establish that the plasma protein is amolecular tein
Wefound that plasma gelsolin differs from cytoplasmic gelsolin principally by the presence of 25 additionalamino acids at its NH2 terminusanddemonstrated that HepG2 cells synthesize both forms of gelsolin, one of which is exported andone of which remains cell-associated
Structural Characterizationof Cytoplasmic and Plasma Gelsolins-Cytoplasmic gelsolin isolated from rabbit lungmacrophagesmigrated onSDS-polyacrylamide gels as asingle polypeptidewith anapparent M, of 90,000, while plasma gelsolin from both man and rabbitmigrated with an apparent
Summary
We have compared the structure of the cytoplasmic and plasma forms of gelsolin and examined their biosynthetic relationships. Plasma form of gelsolin is secreted, and that gelsolins destined for gelsolincontainsan additional peptide of 25 amino intracellularandextracellularcompartmentsare processed acids at its NH2 terminus. Wefound that plasma gelsolin differs from cytoplasmic gelsolin principally by the presence of 25 additionalamino acids at its NH2 terminusanddemonstrated that HepG2 cells synthesize both forms of gelsolin, one of which is exported andone of which remains cell-associated. 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. Amino Acid and NH,-terminal Sequence Analysis-Rabbit macrophage and human plasma gelsolins which were more th a n 9 8 5 pure The NH2-terminal amino acid sequencesof cyanogen bromide-cleavedgelsolin fragments were determined after the peptideswere separated by gel electrophoresis and recovered from polyacrylamide gels by electrodialysis
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