A SOLUBLE-PROTEIN IS IMPORTED INTO EUGLENA CHLOROPLASTS AS A MEMBRANE-BOUND PRECURSOR

The Euglena precursor to the small subunit of ribulose-l,5-bisphosphat e carboxylase/oxygenase (pSSU) is a polyprotein. To determine the tran sport route from cytoplasm to chloroplast, Euglena was pulse labeled w ith S-35-sulfate and the organelles were separated on sucrose gradient s. After a pulse, pSSU was found in the endoplasmic reticulum (ER) and Golgi apparatus. During a chase, ER- and Golgi-localized pSSU decreas ed concomitant with the appearance of SSU in chloroplasts. SSU was not found in pSSU-containing ER and Golgi fractions. Na2CO3 did not remov e pSSU from ER or Golgi membranes, indicating that it was an integral membrane protein. pSSU was inserted in vitro into canine microsomes, a nd Na2CO3 did not remove pSSU from the microsomal membrane. The in viv o and in vitro experiments show that Euglena pSSU is inserted into the ER membrane and transported as an integral membrane protein to the Go lgi apparatus before chloroplast import and polyprotein processing.