Ultrarapid endocytotic uptake of large molecules in Dunaliella species

This paper describes the uptake of Lucifer Yellow carbohydrazide and fluorescent dextrans labeled with fluorescein isothiocyanate or Sodium Green (molecular masses ranging from 522 to 2 x 10(6) Da) by Dunaliella spp. halotolerant unicellular green algae isolated from salt pools in the Sinai peninsula. The fluorescent dyes were taken up into a set of vesicles around the nucleus and just above the chloroplast. It proved impossible to inhibit uptake of the fluorescent compounds in cells treated with a large variety of metabolic and other inhibitors. Cell labeling was complete within half a minute of addition of fluorescent compounds to the outside medium; efflux was equally rapid. The results are interpreted in terms of an endocytotic process whereby the outside medium, together with any substance dissolved in it, remains within vesicles enclosed within the cell body but cycles rapidly between the plasma membrane and the interior of the cell. The outside medium does not pass across the vesicular membrane, nor enters the cytosol.