FIG. 1. Schematic representation of the MPRs. 0, N-linked glycosylation

13 of the CI-MPR represents the region that is similar to the type I1 repeats of fibronectin. Modified from Ref. 14. type of the cells that lack endogenous CI-MPR can be corrected by transfection with a CI-MPR cDNA (28, 29). The residual sorting found in the CI-MPR defective cells appears to be mediated by the CD-MPR. Supporting this, when such cells are treated with an anti-CD-MPR antiserum, secretion of lysosomal enzymes is increased (27). These results indicate that both receptors function in lysosomal enzyme sorting, with the CI-MPR handling most of the traffic. One clear difference between the two receptors relates to the endocytosis of extracellular lysosomal enzymes. While both receptors cycle to the plasma membrane (30), only the CI-MPR is capable of binding and internalizing lysosomal enzymes (27). This difference, however, does not resolve the fundamental question of why two separate MPRs exist.