CELL-FREE TRANSFER AND SORTING OF MEMBRANE-LIPIDS IN SPINACH - DONOR AND ACCEPTOR SPECIFICITY

The donor and acceptor specificity of cell-free transfer of radiolabeled membrane constituents, chiefly lipids, was examined using purified fractions of endoplasmic reticulum, Golgi apparatus, nuclei, plasma membrane, tonoplast, mitochondria, and chloroplasts prepared from green leaves of spinach. Donor membranes were radiolabeled with [C-14]acetate. Acceptor membranes were unlabeled and immobilized on nitrocellulose filters. The assay was designed to measure membrane transfer resulting from ATP- and temperature-dependent formation of transfer vesicles by the donor fraction in solution and subsequent attachment and/or fusion of the transfer vesicles with the immobilized acceptor. When applied to the analysis of spinach fractions, significant ATP-dependent transfer in the presence of cytosol was observed only with endoplasmic reticulum as donor and Golgi apparatus as acceptor. Transfer in the reverse direction, from Golgi apparatus to endoplasmic reticulum, was only 0.2 to 0.3 that from endoplasmic reticulum to Golgi apparatus. ATP-dependent transfers also were indicated between nuclei and Golgi apparatus from regression analysis of transfer kinetics. Specific transfer between Golgi apparatus and plasma membrane and, to a lesser extent, from plasma membrane to Golgi apparatus was observed at 25-degrees-C compared to 4-degrees-C but was not ATP plus cytosol-dependent. All other combinations of organelles and membranes exhibited no ATP plus cytosol-dependent transfer and only small increments of specific transfer comparing transfer at 37-degrees-C to transfer at 4-degrees-C. Thus, the only combinations of membranes capable of significant cell-free transfer in vitro were those observed by electron microscopy of cells and tissues to be involved in vesicular transport in vivo (endoplasmic reticulum, Golgi apparatus, plasma membrane, nuclear envelope). Of these, only with endoplasmic reticulum (or nuclear envelope) and Golgi apparatus, where transfer in situ is via 50 to 70 nm transition vesicles, was temperature- and ATP-dependent transfer of acetate-labeled membrane reproduced in vitro. Lipids transferred included phospholipids, mono- and diacylglycerols, and sterols but not triacylglycerols or steryl esters, raising the possibility of lipid sorting or processing to exclude transfer of triacylglycerols and steryl esters at the endoplasmic reticulum to Golgi apparatus step.