Multiple internalization motifs differentially used by prolactin receptor isoforms mediate similar endocytic pathways

Prolactin (PRL) regulates a variety of physiological processes, including mammary gland growth and differentiation, modulation of behavior, and immune function. A long PRL receptor (IPRLR) and short (sPRLR) isoform were identified in ruminants and rodents, which differ in their distal cytoplasmic domains and possess markedly distinct signaling capacities. Here we compared endocytosis of the bovine isoforms and found that the IPRLR internalized faster than the sPRLR, which would contribute to short-term down-regulation of IPRLR signaling at targets expressing both isoforms. Multiple motifs were required to mediate internalization of the IPRLR, including a phenylalanine (F290) plus a nearby dileucine, and three dileucines proximal to amino acid 272. This is different from the closely related GH receptor that requires only the phenylalanine-containing motif for endocytosis. Truncated IPRLR (cT272), which is the same length as the sPRLR and contained the proximal three dileucines, internalized at the same rate as the full-length IPRLR. Finally, the two dileucines shared by the sPRLR were able to mediate similar endocytic pathways as the IPRLR, as revealed by overexpression of mutant dynamin and clathrin hub, despite the slower rate. These studies define the basis of cellular trafficking of PRLR isoforms and increase our understanding of control of target cell responsiveness by PRL.