Modulation of Phosphate Uptake and Amphotropic Murine Leukemia Virus Entry by Posttranslational Modifications of PIT-2

PIT-2 is a type III sodium phosphate cotransporter and the receptor for amphotropic murine leukemia viruses. We have investigated the expression and the functions of a tagged version of PIT-2 in CHO cells. PIT-2 remained equally abundant at the cell surface within 6 h following variation of the phosphate supply. In contrast, the efficiency of phosphate uptake and retrovirus entry was inversely related to the extracellular phosphate concentration, indicating that PIT-2 activities are modulated by posttranslational modifications of cell surface molecules induced by phosphate. Conformational changes of PIT-2 contribute to both activities, as shown by the inhibitory effect of sulfhydryl reagents known as inhibitors of type II cotransporters. A physical association of PIT-2 with actin was demonstrated. Modifications of the actin network were induced by variations of the concentrations of extracellular phosphate, cytochalasin D, or lysophosphatidic acid. They revealed that the formation of actin stress fibers determines the cell surface distribution of PIT-2, the internalization of the receptor in response to virus binding, and the capacity to process retrovirus entry. Thus, the presence of PIT-2 at the cell surface is not sufficient to ensure phosphate transport and susceptibility to amphotropic retrovirus infection. Further activation of cell surface PIT-2 molecules is required for these functions.