Prolactin receptor gene expression in the rabbit: identification, characterization and tissue distribution of several prolactin receptor messenger RNAs encoding a unique precursor

The prolactin receptor: A cross-species comparison of gene structure, transcriptional regulation, tissue-specificity, and genetic variation

The conserved and multifaceted functions of prolactin (PRL) are coordinated through varied distribution and expression of its cell-surface receptor (PRLR) across a range of tissues and physiological states. The resultant heterogeneous expression of PRLR mRNA and protein across different organs and cell types supports a wide range of PRL-regulated processes including reproduction, lactation, development, and homeostasis. Genetic variation within the PRLR gene also accounts for several phenotypes impacting agricultural production and human pathology. The goal of this review is to highlight the many elements that control differential expression of the PRLR across tissues, and the various phenotypes that exist across species due to variation in the PRLR gene.

Growth hormone increases Stat5 and Stat1 expression in lactating goat mammary gland: a specific effect compared to milking frequency

In ruminants, both milking frequency and exogenous GH treatment affect milk production. In a previous report, we showed that the modulation of milk yield due to variations in milking frequency and GH treatment was associated with variations in mammary cell numbers. The aim of this study was to clarify the different mechanisms governing the effects of GH treatment and milking frequency on signal transducer and activator of transcription (Stat) expression and activation, and on the expression of genes involved in mammary cell differentiation. Six Saanen goats in late lactation were milked once daily from one half-udder and thrice daily from the other half-udder for 23 days. At the same time, the goats were divided into two groups: GH-treated versus control group. After slaughter of the goats, soluble mammary proteins and RNA were extracted from half-udder samples. Levels of Stat5, Stat3 and Stat1 proteins and the Stat activation by phosphorylation were analysed by Western blot. The amounts of Stat5 protein and mRNA were significantly elevated by GH treatment in all half-udders (milked once or thrice daily). Positive Stat5 immunoreactivity was principally localised in the nuclei of epithelial cells, with heterogeneous intensity between cells. No significant changes in Stat5 protein phosphorylation levels were observed. Furthermore, GH significantly increased Stat1 protein levels, without modifying the level of Stat1 tyrosine phosphorylation, and tended to reduce the abundance of Stat3 protein. In contrast, milking frequency failed to modify Stat gene expression, protein level and phosphorylation. Using Northern blot, we showed that levels of kappa casein and prolactin receptor mRNA were not affected by the treatments. These observations suggest that GH probably acts specifically on mammary cells by regulating the expression of Stat1, 3 and 5. In contrast, milking frequency does not act through this regulatory pathway.

Prolactin receptor gene expression in the forebrain of pregnant and lactating rats

Prolactin plays a large role in the onset of maternal behavior at parturition. Knowledge of the change in expression of the prolactin receptor in the brain across pregnancy and lactation, however, is limited. Prolactin receptor gene expression was determined by in situ hybridization histochemistry during pregnancy and lactation in rats. Expression of the mRNA for the longform of the prolactin receptor (PRL-R-L) was measured in various forebrain structures in primigravid rats at different stages of pregnancy, in primiparous rats during early, mid-, and late lactation, and in age-matched, nulliparous females in diestrus. Hybridizations were performed using a [33P]-labeled riboprobe specific for the long form of the prolactin receptor mRNA complimentary to 290 bp of the prolactin receptor gene. The following areas of the forebrain were examined: medial preoptic area (MPOA), median preoptic nucleus both dorsal (MePOd) and ventral (MePOv) to the anterior commissure, ventral lateral septum (LSv), and the ventral and principal parts of the bed nucleus of the stria terminalis (BnSTv and BnSTpr, respectively). Overall, the number of cells expressing PRL-R-L mRNA was significantly higher at 2 h postpartum compared to diestrus in all areas examined except the LSv. In addition, there were lower numbers of PRL-R-L cells during all stages of lactation compared to pregnancy. The number of grains per cell in the MPOA and LSv did not change as dramatically as the number of cells expressing PRL-R-L mRNA in those brain regions. These data contribute to the growing body of evidence that the neural lactogenic system changes as a function of female reproductive state. Changes in PRL-R-L mRNA in terms of behavior and endocrine functions are discussed.

Cloning, characterization, and tissue distribution of prolactin receptor in the sea bream (Sparus aurata)

The prolactin receptor (PRLR) was cloned and its tissue distribution characterized in adults of the protandrous hermaphrodite marine teleost, the sea bream (Sparus aurata). An homologous cDNA probe for sea bream PRLR (sbPRLR) was obtained by RT-PCR using gill mRNA. This probe was used to screen intestine and kidney cDNA libraries from which two overlapping clones (1100 and 2425 bp, respectively) were obtained. These clones had 100% sequence identity in the overlapping region (893 bp) and were used to deduce the complete amino acid sequence of sbPRLR. The receptor spans 2640 bp and encodes a protein of 537 amino acids. Features characteristic of PRLR, two pairs of cysteines, WS box, hydrophobic transmembrane domain, box 1, and box 2, were identified and showed a high degree of sequence identity to PRLRs from other vertebrate species. SbPRLR is 29 and 32% identical to tilapia (Oreochromis niloticus) and goldfish (Carassius auratus) PRLRs, respectively. In the sea bream two PRLR transcripts of 2.8 and 3.2 kb were detected in the intestine, kidney, and gills and a single transcript of 2.8 kb was detected in skin and pituitary by Northern blot. Spermiating gonads (more than 95% male tissue; gonado-somatic index of 0.6) contained, in addition to the 2.8-kb transcript, three more transcripts of 1.9, 1.3, and 1.1 kb. RT-PCR, which is a far more sensitive method than Northern blot, detected PRLR mRNA in gills, intestine, brain, pituitary, kidney, liver, gonads, spleen, head-kidney, heart, muscle, and bone. Immunohistochemistry using specific polyclonal antibodies raised against an oligopeptide from the extracellular domain of sbPRLR detected PRLR in several epithelial tissues of juvenile sea bream, including the anterior gut, renal tubule, choroid membrane of the third ventricle, saccus vasculosus, branchial chloride cells, and branchial cartilage.

Characterization and modulation of a prolactin receptor mRNA isoform in normal and tumoral human breast tissues

The role of prolactin (PRL) and its specific receptor (R-PRL) in human breast tumorigenesis remains unclear. We have investigated here the presence of extracellular-deleted hPRL-R isoforms in normal human breast, fibrocystic disease, primary breast carcinoma (ductal carcinoma, ductulo-lobular and lobular) and breast cancer cell lines (T47-D and MCF-7). RT-PCR and Southern blot analysis demonstrated the expression of full-length hPRL-R transcript in all samples tested. We also detected a hPRL-R transcript generated by alternative exon 6 splicing. This isoform has a 170 bp deletion in its extracellular sub-domain that induces a frameshift. Thus, the predicted amino-acid sequence should encode a putative soluble protein with the N-terminal sub-domain of the hPRL-R and 10 additional carboxy-terminal residues. This isoform should not bind PRL as previously demonstrated by other experiments. Moreover, the ratio of full-length to deleted form of hPRL-R transcripts differs from normal to tumoral breast tissue. This ratio is higher in tumoral mammary gland than in normal tissue. Our data suggest that the alternative splicing of the hPRL-R gene towards the deleted transcript may be a mechanism to down- or up-regulate the expression of the native transcript of hPRL-R in accordance to the physiological or pathological state of the mammary gland.

Cloning and sequence analysis of the extracellular region of the polar bear (Ursus maritimus) luteinizing hormone receptor (LHr), follicle stimulating hormone receptor (FSHr), and prolactin receptor (PRLr) genes and their expression in the testis of the black bear (Ursus americanus)

Male black bears undergo seasonal changes in testicular activity. The testes are fully functional from May through July, regress from July through December, and recrudesce from January until May. The mechanisms responsible for the initiation of testicular recrudescence in the bear are unknown. The objectives of this study were to: (1) clone and sequence a substantial fragment of the extracellular portion of the luteinizing hormone receptor (LHr: 646 bp) and follicle stimulating hormone receptor (FSHr: 852 bp), and the extracellular/transmembrane portion of the prolactin receptor (PRLr: 680 bp) in the bear using reverse transcription-polymerase chain reaction (RT-PCR); and (2) determine whether the expression of LH-, FSH-, and PRL-receptor mRNA transcripts differs between the beginning and terminal stages of testicular recrudescence. Comparisons of the partial cDNA and predicted amino acid sequences of ursine receptors with the corresponding sequences from the pig, cow, human, and rat suggest that the LHr and FSHr are highly conserved (LHr: 87.1-93.7%; FSHr: 86.0-92.7%) whereas the PRLr is less well conserved (81-87%). Testicular LHr mRNA was more abundant during the breeding season in May than during the non-breeding season (early stage of recrudescence) in January. In contrast, testicular FSHr mRNA abundance was greater in January than in May. Testicular PRLr mRNA appeared equally abundant in January and May; however, two additional transcripts were present during the breeding season in May. This study provides molecular tools for future investigations of the control of testicular recrudescence in the black bear and demonstrates that the expression of testicular gonadotropin and PRL receptor mRNA is seasonally regulated. Mol. Reprod. Dev. 55:136-145, 2000.

Characterization of unique truncated prolactin receptor transcripts, corresponding to the intracellular domain, in the testis of the sexually mature chicken

We have examined expression of the chicken PRL receptor (cPRLR) gene in different tissues of the chicken by Northern blot analysis. Most tissues examined (ovary, testis, oviduct, kidney, and fat) possess a prominent full-length (4.6-kb) cPRLR transcript. A larger (11.7-kb) transcript is also detected in ovary, oviduct, testis, and kidney after longer exposure. A unique pattern of cPRLR expression was found in the testis of sexually mature chickens, which have an unusually high abundance of three small transcripts (1.2, 1.7, and 2 kb) in addition to the 4.6-kb transcript found in other tissues. Three domain-specific complementary DNA (cDNA) probes were constructed that correspond to the first and second ligand-binding regions in the extracellular domain and the transmembrane-intracellular domain. With these probes, Northern blot analysis of polyadenylated RNA prepared from the testes of a mature (22-week-old) chicken indicates that the highly abundant (1.2- and 1.7-kb) and less abundant (2.0-kb) cPRLR transcripts in testis hybridize only to the intracellular domain probe. Two types of truncated testis-specific cPRLR transcripts were identified using 5'-RACE (rapid amplification of cDNA ends) analysis of polyadenylated RNA from the testis of a 22-week-old chicken. The predominant truncated cDNA sequence contains the highly conserved box 1 motif [(+)box 1 cDNA] and diverges (at nucleotide 1396) from that of the cPRLR cDNA, just downstream of the transmembrane domain. The other truncated cDNA lacks the box 1 motif [(-)box 1 cDNA], which is replaced by 39 bases that could encode a hydrophobic N-terminus with a putative 5'-untranslated region of 131 bases. Young chickens predominately express the full-length cPRLR messenger RNA (4.6 kb) in the testis. At the onset of sexual maturity, there is a dramatic increase in abundance of the testis-specific (+)box 1 transcript, whereas expression of the full-length cPRLR is depressed. The presence of truncated [(+) or (-)box 1] cPRLR transcripts in the sexually mature chicken testis suggests a complex mechanism of PRL action on gonadal function.

Luteotropic hormone receptors in the ovary of the mink (Mustela vison) during delayed implantation and early-postimplantation gestation

The reproductive cycle of the mink displays rigid seasonality and obligate embryonic diapause. After ovulation, the corpus luteum (CL) involutes, and it secretes basal progesterone until activated prior to implantation. To study changes in the relative abundance of luteal prolactin and LH receptor mRNA through gestation, ovaries and serum were collected from pregnant female mink at 2-day intervals (n = 3 per date) through embryonic diapause and CL activation (March 19-31) and at 5-day intervals through implantation and early-postimplantation gestation (March 31-April 15). To determine the effect of endogenous prolactin, mink received Alzet osmotic minipumps releasing 2 mg/day 2-bromo-alpha-ergocryptine (bromocriptine) or saline on March 19. Ovaries and serum were taken from 3 animals every 2 days until March 31. Prolactin receptor mRNA in ovaries was low during CL activation but increased 3-fold through embryo implantation. Its abundance correlated with prolactin binding to ovarian membranes and with circulating prolactin. Bromocriptine suppressed endogenous prolactin levels and prevented the increase in prolactin receptor mRNA. There was a transient peak in LH receptor mRNA in the ovaries at March 19-23, which declined to basal levels by March 25 and remained constant through midgestation. Bromocriptine prevented the preimplantation peak in LH receptor mRNA and reduced its abundance below pretreatment levels. The results suggest that prolactin up-regulates its receptor and maintains the LH receptor in the mink CL. The pattern of LH receptor mRNA argues for a role for LH in CL reactivation and termination of embryonic diapause in mink.

Stoichiometric structure-function analysis of the prolactin receptor signaling domain by receptor chimeras

The intracellular domain of the prolactin (PRL) receptor (PRLr) is required for PRL-induced signaling and proliferation. To identify and test the functional stoichiometry of those PRLr motifs required for transduction and growth, chimeras consisting of the extracellular domain of either the alpha or beta subunit of human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GM-CSFr) and the intracellular domain of the rat PRLr were synthesized. Because the high-affinity binding of GM-CSF results from the specific pairing of one alpha- and one beta-GM-CSFr, use of GM-CSFr/PRLr chimera enabled targeted dimerization of the PRLr intracellular domain. To that end, the extracellular domains of the alpha- and beta-GM-CSFr were conjugated to one of the following mutations: (i) PRLr C-terminal truncations, termed alpha278, alpha294, alpha300, alpha322, or beta322; (ii) PRLr tyrosine replacements, termed Y309F, Y382F, or Y309+382F; or, (iii) PRLr wild-type short, intermediate, or long isoforms. These chimeras were cotransfected into the cytokine-responsive Ba/F3 line, and their expression was confirmed by ligand binding and Northern and Western blot analyses. Data from these studies revealed that heterodimeric complexes of the wild type with C-terminal truncation mutants of the PRLr intracellular domain were incapable of ligand-induced signaling or proliferation. Replacement of any single tyrosine residue (Y309F or Y382F) in the dimerized PRLr complex resulted in a moderate reduction of receptor-associated Jak2 activation and proliferation. In contrast, trans replacement of these residues (i.e., alphaY309F and betaY382F) markedly reduced ligand-driven Jak2 activation and proliferation, while cis replacement of both tyrosine residues in a single intracellular domain (i.e., alphaY309+382F) produced an inactive signaling complex. Analysis of these GM-CSFr-PRLr complexes revealed equivalent levels of Jak2 in association with the mutant receptor chains, suggesting that the tyrosine residues at 309 and 382 do not contribute to Jak association, but instead to its activation. Heterodimeric pairings of the intracellular domains from the known PRLr receptor isoforms (short-intermediate, short-long, and intermediate-long) also yielded inactive receptor complexes. These data demonstrate that the tyrosine residues at 309 and 382, as well as additional residues within the C terminus of the dimerized PRLr complex, contribute to PRL-driven signaling and proliferation. Furthermore, these findings indicate a functional requirement for the pairing of Y309 and Y382 in trans within the dimerized receptor complex.

Corticosterone is required for the prolactin receptor gene expression in the late pregnant mouse mammary gland

In order to clarify the prolactin receptor (PRL-R) gene expression at lactogenesis, the levels of the long and short forms of PRL-R mRNA were determined by the competitive RT-PCR in the pregnant, lactating and ovariectomized midpregnant mouse mammary gland. Plasma concentrations of corticosterone and progesterone were determined by RIA. The long form of PRL-R mRNA level was low until 10:00 on day 18, increased 3.3-fold at 22:00 on day 18 of pregnancy and further increased to 4.6-fold at 10:00 on day 0 of lactation. The short form of PRL-R mRNA level remained unchanged during this time period. The corticosterone:progesterone ratio increased 15.5-fold during the last 1.5 days of pregnancy. Corticosterone increased the long form of PRL-R mRNA level when the tissues on day 17 were cultured. On day 12 of pregnancy and following ovariectomy, corticosterone was exceedingly high from 2 h to 8 h and the corticosterone:progesterone ratio changed prior to the increase in the long form of PRL-R mRNA level. We conclude that corticosterone increases the PRL-R gene expression in the mammary gland before the onset of parturition.

Removal of milk by suckling acutely increases the prolactin receptor gene expression in the lactating mouse mammary gland

To determine the effect of suckling on the prolactin receptor (PRL-R) gene expression, we measured the quantity of PRL-R mRNA in the lactating mouse mammary gland. When the pups were separated from their mother on day 5 of lactation, the long form of PRL-R (PRL-R[L]) mRNA disappeared with a half-life of 12.5 h for the first 9 h and 3.0 h for the following 9-15 h. By supplying pups to mice which had been weaned for 24 h, PRL-R(L) mRNA increased 2.5-fold during the next 6 h-period. The increase in PRL-R(L) mRNA was found in the mammary glands from which the pups removed milk. The number of mammary PRL-R protein decreased or increased following weaning or following the removal of milk by suckling, respectively. From these observations. it was concluded that the removal of milk acutely increases the level of PRL-R(L) mRNA during lactation.

Up-regulation of the human prolactin receptor in the endometrium

In humans, uterine endometrial stromal cells differentiate (decidualize) into decidual cells that express prolactin (PRL). Decidual PRL expression continues throughout pregnancy, thus decidual cells lining fetal membranes of term placenta synthesize and secrete PRL. To examine the hypothesis that PRL may play an autocrine role in the decidual cells, we examined the expression of the PRL receptor (PRL-R) during in vitro decidualization of stromal cells and in term decidua. In endometrial stromal cells decidualized by treatment with 1 μM medroxyprogesterone and 10 nM estradiol for 3, 6, and 9 d, respectively, a 12.7 kb PRL-R transcript increased 3-3.5-fold, 16.5-17-fold, and 23.5-24-fold, respectively, compared with untreated controls, in duplicate experiments. Progesterone-dependent PRL-R and PRL expression were stimulated by 1 μ/M prostaglandin E(2). Term decidua expressed the long form of the PRL-R and five major PRL-R transcripts (12.7, 9.7, 7.0, 3.6, and 2.8 kb). In contrast, human liver expressed two major transcripts (12.7 and 9.7 kb) while hepG2 cells expressed a single 7.0-kb-sized transcript. These studies demonstrate that PRL-R expression is stimulated upon progesterone-induced PRL gene expression in endometrial stromal cells supporting the hypothesis that PRL may have an autocrine effect in the endometrium and decidua.

Expression and regulation of growth hormone (GH) and prolactin (PRL) receptors in a rat insulin producing cell line (INS-1)

We have examined the expression of growth hormone (GH) and prolactin (PRL) receptors, and the binding parameters of human GH (hGH), on the rat insulin producing cell line INS-1. We found that, like normal insulin producing beta-cells, INS-1 cells express both GH and PRL receptors, and the majority of human GH (hGH) binding sites on this cell line are of lactogenic specificity. As calculated from Scatchard plots, about 6600 hGH binding sites with a Ka of 6.2 x 10(8) M-1 are present per cell. Northern blot analysis showed two mRNA species of 4 and 1.6 kb for the GH receptor and, one major species of 10.5 kb for the PRL receptor. The PRL receptor mRNA was up-regulated by bovine GH (bGH), rat PRL (rPRL), in a time- and dose-dependent manner. On the contrary, bGH and rPRL, down-regulated the expression of GH receptor gene. The importance of this differential regulation in in vivo and in vitro studies are discussed.

Prolactin induces growth inhibition and promotes differentiation of CHO cells stably transfected with prolactin receptor complementary DNA

We have characterized a stable and functional transfectant of the rabbit prolactin receptor in Chinese hamster ovary cells, and investigated the action of prolactin (PRL) on the growth and differentiation of this transfectant (clone E32). PRL induced a significant inhibition of E32 cell proliferation. Growth inhibition correlated with gene induction of the molecular marker of ovarian differentiation cholesterol side chain cleavage P450 (P450scc). Both effects were inversely proportional to cell confluence. The limits and potential development of such transfected cellular systems are discussed.

Molecular cloning of the bovine prolactin receptor and distribution of prolactin and growth hormone receptor transcripts in fetal and utero-placental tissues

We have isolated a bovine prolactin (bPRL) receptor cDNA from an endometrial cDNA library, which predicts a 557 amino acid transmembrane protein similar to the long forms of other characterized prolactin receptors. The predicted cytoplasmic domain is slightly truncated primarily by a stop codon located 36 codons 5' from the stop utilized in the human hepatic transcript. When expressed in COS cells, this cDNA was shown to encode a protein which bound bovine placental lactogen (bPL) and bPRL with nearly equal affinity (KD for bPL, 2.03 x 10(-10) M; bPRL, 3.07 x 10(-10) M). Northern analysis demonstrated multiple transcripts, with maternal liver, corpus luteum, intestine, endometrium and fetal liver containing a major transcript of about 3.8 kb, and maternal corpus luteum and endometrium, a second sized transcript of apparently equal abundance of 4.4 kb. This difference did not appear to be within the coding region. Primer extension analysis of maternal hepatic and endometrial transcripts revealed considerable heterogeneity. Examination of the distribution of prolactin and growth hormone receptor transcripts at mid-pregnancy by semi-quantitative reverse transcriptase polymerase chain reaction showed that both are widespread in bovine fetal and placental tissues. This isolation of bovine prolactin receptor cDNA, and description of receptor distribution will facilitate study of the action of the placental and pituitary members of this gene family during pregnancy.

Prolactin as an immunoregulatory hormone in mammals and birds

The immunoregulatory function of prolactin (PRL) and the mechanism of its action in mammals seem to be well documented. Reciprocal interdependence between PRL secretion and immune system function is essential for normal ontogeny, development and aging. PRL receptors in lymphocytes participate in the transduction of its regulatory signal into the intracellular enzymatic machinery including that of the nucleus, leading to the expression of some genes and to the synthesis of new proteins. Activation of phosphoinositide turnover and subsequent increase in protein kinase-C activity seems to be a possible mechanism acting in the regulatory influence of PRL on mammalian immune cells. These cells in turn, under mitogen or antigen stimulation, secrete a substance with PRL-like activity. The regulatory function of PRL within the avian immune system is less well known, but it seems to have some features in common with those in mammals. Direct mitogenic action on thymocytes and splenocytes in the chicken might indicate the existence of PRL receptors in these cells and could explain the immunostimulatory effect of PRL observed in vivo, which is dependent on the time of hormone administration. As the avian PRL stimulates mitogenesis of rat Nb2 lymphoma cells, the mechanism of direct PRL action on immune cells in mammals and birds seems to be similar. PRL in chickens also modifies the level and the diurnal rhythm of corticosterone which, in turn, influences the immunoregulatory effect exerted by PRL. Thus, PRL seems to be an important factor, influencing directly or indirectly the avian immune system.

Identification of a novel growth hormone binding protein mRNA in rat liver

The presence of highly specific serum binding proteins for growth hormone (GH) has been well characterized in many species. In the rat the major growth hormone-binding protein (GH-BP) is a truncated, variant form of the target tissue GH receptor and is derived by an alternative mRNA splicing event. The GHBP mRNA is coexpressed in all tissues expressing the full length GH receptor. In the present study, we have made an oligonucleotide probe to the unique hydrophilic tail of the rat GHBP mRNA (1.2kb) and identified a novel GHBP-like mRNA of 2.6 kb transcript in addition to the 1.2 kb transcript. This unique 2.6 kb transcript was expressed/detected only in rat liver. There was no significant difference in abundance between the sexes or during pregnancy, implying that this transcript may be regulated independently of the 1.2 kb mRNA. The 2.6 kb transcript was clearly identifiable in the fetus, as was the 1.2 kb transcript, but showed virtually no change in abundance with age, in sharp contrast to the 1.2 kb mRNA, which has a distinct developmental pattern, being low in the fetus and peaking early postnatally. RNAse H treatment suggested that this 2.6 kb transcript is polyadenylated. A corresponding 2.6 kb mRNA has been detected using a longer cDNA or cRNA probe for the GH-binding domain of the rat GHR/GHBP. These data collectively suggest that the 2.6 kb mRNA transcript is a bonafide but tissue-specific GHBP mRNA and that the 1.2 and 2.6 kb mRNAs are likely to differ primarily with respect to the length of the 3' untranslated region of the sequence.

Identification of prolactin and growth hormone binding proteins in rabbit milk

Two distinct soluble proteins that specifically bind 125I-labeled human growth hormone (GH) are identified in the supernatant of ultracentrifuged rabbit milk, using HPLC gel filtration. The higher molecular weight proteins is GH specific, whereas the other one is specific for prolactin (PRL). The PRL-binding protein has a very high affinity for the hormone, almost 10 times higher than the affinity of the mammary gland membrane receptor. The PRL-binding protein is immunoprecipitated by a monoclonal antibody against the PRL receptor; another monoclonal antibody, which inhibits the PRL binding to mammary gland membranes, is a poor competitor for the PRL binding to the milk protein. These findings suggest that the milk PRL-binding protein corresponds to the binding domain of the receptor, but also that the conformation of the receptor and of the binding protein might differ. The milk and the plasma GH-binding proteins have a similar binding affinity. In cross-linking experiments using 125I-labeled human GH, the Mr of the GH-binding protein and of the PRL-binding protein were estimated to be 51,000 and 33,000, respectively. The binding proteins identified in the present work are probably responsible for the transport of their specific ligands in the milk. It is also conceivable that they have a role in the effects of GH and PRL in the mammary gland and/or the intestine of the young.