A soluble protein is imported into Euglena chloroplasts as a membrane-bound precursor.
The Euglena precursor to the small subunit of ribulose-15-bisphosphate carboxylase/oxygenase (pSSU) is a polyprotein. To determine the transport route from cytoplasm to chloroplast, Euglena was pulse labeled with 35S-sulfate and the organelles were separated on sucrose gradients. After a pulse, pSSU was found in the endoplasmic reticulum (ER) and Golgi apparatus. During a chase, ER-and Golgi-localized pSSU decreased concomitant with the appearance of SSU in chloroplasts. SSU was not found in pSSU-containing ER and Golgi fractions. Na2CO3 did not remove pSSU from ER or Golgi membranes, indicating that it was an integral membrane protein. pSSU was inserted in vitro into canine microsomes, and Na2CO3 did not remove pSSU from the microsomal membrane. The in vivo and in vitro experiments show that Euglena pSSU is inserted into the ER membrane and transported as an integral membrane protein to the Golgi apparatus before chloroplast import and polyprotein processing.
Sulli
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School of Biological Sciences, University of Nebraska, Lincoln 68588, USA.
Schwartzbach
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SD
D000818:Animals; D001483:Base Sequence; D001692:Biological Transport; D002458:Cell Fractionation; D002500:Centrifugation, Isopycnic; D002736:Chloroplasts / Q000378:metabolism*; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005056:Euglena gracilis / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism*; D008861:Microsomes / Q000378:metabolism; D008969:Molecular Sequence Data; D011233:Precipitin Tests; D011498:Protein Precursors / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D021382:Protein Sorting Signals / Q000378:metabolism; D012273:Ribulose-Bisphosphate Carboxylase / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D013045:Species Specificity
D018076:DNA, Complementary; D011498:Protein Precursors; D021382:Protein Sorting Signals; D012273:Ribulose-Bisphosphate Carboxylase