Cleavage of the alpha 1-microglobulin-bikunin precursor is localized to the Golgi apparatus of rat liver cells.
alpha 1-Microglobulin, a plasma protein with immunoregulatory properties, and bikunin, the light chain of the proteinase inhibitors inter-alpha-inhibitor and pre-alpha-inhibitor, are translated as a precursor protein from the same mRNA. The cosynthesis of alpha 1-microglobulin and bikunin is unique compared to other proproteins such as procomplement components and prohormones, since alpha 1-microglobulin and bikunin have no known functional connection. Different forms of intracellular rat liver alpha 1-microglobulin were isolated and characterized by amino acid sequence analysis, lectin binding and glycosidase treatment. Their subcellular distribution was studied by Nycodenz and sucrose gradient centrifugation, pulse-chase experiments, and electrophoresis with subsequent immunoblotting, using pro-C3 and prohaptoglobin as reference proteins. Two alpha 1-microglobulin-bikunin precursors (40 and 42 kDa), containing one and two N-linked oligosaccharides, respectively, were detected in the endoplasmic reticulum. After transport to the Golgi apparatus, the precursors were cleaved, probably C-terminal to the sequence Arg-Ala-Arg-Arg immediately preceding the bikunin part, yielding free sialylated 28 kDa alpha 1-microglobulin, representing the mature protein. The cleavage was almost complete in phosphatidylinositol 4-kinase-enriched membranes, previously identified as a post-Golgi compartment. A fourth intracellular form of alpha 1-microglobulin, 26 kDa, lacked sialic acid. None of the intracellular forms carried the yellow-brown chromophore associated with alpha 1-microglobulin when purified from serum and urine, suggesting that this chromophore becomes linked to the protein after its secretion from the liver cells.
Bratt
T
T
Department of Medical and Physiological Chemistry, University of Lund, Sweden.
Olsson
H
H
Sjöberg
E M
EM
Jergil
B
B
Akerström
B
B
D019870:1-Phosphatidylinositol 4-Kinase; D000510:Alpha-Globulins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D000818:Animals; D002478:Cells, Cultured; D006023:Glycoproteins / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D008562:Membrane Glycoproteins; D010770:Phosphotransferases / Q000378:metabolism; D011498:Protein Precursors / Q000737:chemistry / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013347:Subcellular Fractions / Q000378:metabolism; D014360:Trypsin Inhibitor, Kunitz Soybean
D000510:Alpha-Globulins; D006023:Glycoproteins; D008562:Membrane Glycoproteins; D011498:Protein Precursors; C109148:SPINT2 protein, human; C016001:alpha-1-microglobulin; D014360:Trypsin Inhibitor, Kunitz Soybean; D010770:Phosphotransferases; D019870:1-Phosphatidylinositol 4-Kinase