In vitro uptake and processing of maize auxin-binding proteins by ER-derived microsomes

We have analyzed auxin-binding proteins from maize encoded by the Zm-ERabp gene family. Open reading frames of cDNA clones predict proteins containing N-terminal hydrophobic signal sequences. In vitro studies show that the Zm-ERabp1 protein can be translocated into ER-derived microsomes where it is processed and glycosylated. A cDNA clone encoding the Zm-ERabp4 protein predicts an open reading frame with a signal sequence that shows striking differences in charge distribution, in comparison to the signal sequence of Zm-ERabp1. Two translation products are synthesized from the Zm-ERabp4 transcript in the in vitro system, but only one of them is translocated into maize endosperm microsomes, indicating that specific cotranslational modifications in the primary sequence remaining after processing may play a role in the cellular trafficking of the Zm-ERabp4 protein.