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Prolactin-binding components in rabbit mammary gland: characterization by partial purification and affinity labeling.

The molecular characteristics of the PRL receptor isolated from rabbit mammary gland microsomes were investigated. Two approaches were employed: 1) affinity purification of PRL receptors and direct electrophoretic analysis, and 2) affinity cross-linking of microsomal receptors with [125I]ovine PRL [( 125I]oPRL). PRL receptors were solubilized from mammary microsomes with 3-[( 3-cholamidopropyl)dimethylammonio]1-propane sulfonate and purified using an oPRL agarose affinity column. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining of the gel revealed at least nine bands, including a 32,000 mol wt band which was most intensively labeled with 125I using the chloramine-T method. Covalent labeling of PRL receptors with [125I]oPRL was performed using N-hydroxysuccinimidyl-4-azido benzoate, disuccinimidyl suberate, or ethylene glycol bis (succinimidyl succinate). A single band of 59,000 mol wt was produced by all three cross-linkers when sodium dodecylsulfate-polyacrylamide gel electrophoresis was performed under reducing conditions. Assuming 1:1 binding of hormone and binding subunit and by subtracting the mol wt of [125I]oPRL, which was estimated from the migration distance on the gel, the mol wt of the binding subunit was calculated as 32,000. In the absence of dithiothreitol during electrophoresis, only one major hormone-receptor complex band was observed. The same mol wt binding components were also detected in microsomal fractions of rabbit kidney, ovary, and adrenal. A slightly higher mol wt binding subunit was observed in rat liver microsomes. Rabbit liver microsomes revealed five [125I]oPRL-binding components, three of which were considered to be those of a GH receptor. Moreover, affinity labeling of detergent-solubilized and affinity purified mammary PRL receptors showed a similar major binding subunit. From these observations, we conclude that this predominant 32,000 mol wt component is a major binding subunit of the PRL receptor molecule and does not aggregate with itself or with other subunits through S-S linkages.