XPR1 is a major phosphate transporter on platelets. (A) Data from the Human Protein Atlas project and GTEx Analysis V7 (https://gtexportal.org) were used to determine the expression of transcripts encoding for the 10 reported phosphate transporters in bone marrow and whole blood. Platelet Web (http://plateletweb.bioapps.biozentrum.uni-wuerzburg.de/plateletweb.php) and PubMed were used to establish whether protein expression on platelets had been described. TPM, transcripts per million. Cell membranes from the indicated number of human (B) or mouse (C) platelets were analyzed by western blot, using antibodies directed against the XPR1 N- or C-terminal portions. Lysed HEK293 cells transiently transfected with a pCHIX-XPR1–coding plasmid were loaded as a positive control (XPR1). (D) Bright-field image (left), confocal laser scanning image (middle; green) of XPR1 staining in nonpermeabilized resting human platelets, and merged image (right). Scale bar, 1 µm. (E) Transmission electron microscopy of immunogold-stained (10-nm particles) XPR1 on human platelets. Open canalicular system (OCS), dense tubular system (DTS), and α-granules are shown. Arrows indicate XPR1; scale bar, 100 nm.