Cells of Chlorella vulgaris induced for hexose transport contain a membrane-bound protein component that is missing in non-induced cells. This was shown by double labelling experiments with [14C]phenylalanine and [3H]phenylalanine applying the method of Kolber and Stein [Nature (Lond) 209 (1966) 691]. The specific protein is completely absent from soluble fractions. 2. An enrichment of the double-labelled peak was observed in membrane fractions enriched in plasmalemma. The best purification was obtained when cell walls were purified; the residual membranes attached to the walls contained the transport protein with a 12-times-higher specific activity than the crude extract. 3. The transport protein had the characteristics of an intrinsic membrane protein. Its molecular weight was found to be 30 000 on sodium dodecylsulphate gels. The protein did not show sugar binding activity, however. 4. Induced cells lose their state of induction with a half life of about 4 h; 7 h after induction the double-labelled transport protein is no longer detectable.
