FIGURE 6.

SR-BI functionality is enhanced in the presence of PCPE2. A, a histogram of PCPE2 cell surface expression by flow cytometry. CHO cells were either mock-transfected or control CHO cells (dotted line) or transiently transfected with PCPE2 (shaded solid line). The next day the cells were stained for surface PCPE2 expression as described under “Experimental Procedures.” A 2-fold increase (47926 ± 1025 versus 21336 ± 525, mean ± S.D., n = 3 independent experiments) in PCPE2 surface expression in transiently transfected cells is consistent with the PCPE2 known location in the extracellular matrix. B, the uptake of ³H-COE-labeled HDL by control and PCPE2-overexpressing CHO cells expressed as ng of HDL taken up/mg of cell protein. Confluent wells of CHO cells were either mock-transfected or transfected with a plasmid encoding PCPE2 as described under “Experimental Procedures.” The next day the cells were washed and then incubated for 1.5 h with 10 μg of ³H-COE-labeled HDL obtained from either diet-fed LDLr^−/− or LDLr^−/−, PCPE2^−/− mouse plasma as described under “Experimental Procedures.” The cells were washed extensively, and then the cell pellet was digested in 0.1 n NaOH and its protein and radioactivity content quantified as described under “Experimental Procedures.” Each value represents the mean ± S.D. of n = 3 wells/condition from three independent experiments. C, an immunoblot of SR-BI and PCPE2 protein expression from control and PCPE2-overexpressing CHO cells. Each lane contains 25 μg of total protein. D, a histogram of SR-BI surface expression in mock (dotted line)- and PCPE2-transfected CHO (shaded solid line) cells by flow cytometry. Overexpression of PCPE2 leads to a shift to a single population, with a significantly higher frequency of SR-BI per cell (50992 ± 1070 versus 36244 ± 905, mean ± S.D., n = 3 independent experiments).