Assembly and secretion of fibrinogen. Degradation of individual chains

J Biol Chem. 1992 Nov 15;267(32):23151-8.

Abstract

Hep G2 cells produce surplus A alpha and gamma fibrinogen chains. These excess chains, which are not secreted, exist primarily as free gamma chains and as an A alpha-gamma complex. We have determined the intracellular location and the degradative fate of these polypeptides by treatment with endoglycosidase-H and by inhibiting lysosomal enzyme activity, using NH4Cl, chloroquine, and leupeptin. Free gamma chain and the gamma component of A alpha-gamma are both cleaved by endoglycosidase-H, indicating that the gamma chains accumulate in a pre-Golgi compartment. Lysosomal enzyme inhibitors did not affect the disappearance of free gamma chains but inhibited A alpha-gamma by 50%, suggesting that A alpha-gamma is degraded in lysosomes. The degradative fate of individual chains was determined in transfected COS cells which express but do not secrete single chains. Leupeptin did not affect B beta chain degradation, had very little affect on gamma chain, but markedly inhibited A alpha chain degradation. Antibody to immunoglobulin heavy chain-binding protein (GRP 78) co-immunoprecipitated B beta but not A alpha or gamma chains. Preferential binding of heavy chain-binding protein to B beta was also noted in Hep G2 cells and in chicken hepatocytes. Taken together these studies indicate that B beta and gamma chains are degraded in the endoplasmic reticulum, but only B beta is bound to BiP. By contrast A alpha chains and the A alpha-gamma complex undergo lysosomal degradation.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Ammonium Chloride / pharmacology
  • Animals
  • Cell Line
  • Chickens
  • Chloroquine / pharmacology
  • Fibrinogen / biosynthesis
  • Fibrinogen / metabolism*
  • Humans
  • Kinetics
  • Leupeptins / pharmacology
  • Liver / metabolism
  • Lysosomes / drug effects
  • Lysosomes / enzymology
  • Macromolecular Substances
  • Methionine / metabolism
  • Sulfur Radioisotopes
  • Transfection
  • Tumor Cells, Cultured

Substances

  • Leupeptins
  • Macromolecular Substances
  • Sulfur Radioisotopes
  • Ammonium Chloride
  • Chloroquine
  • Fibrinogen
  • Methionine
  • leupeptin