Binding characteristics and affinity labeling of protein constituents of the human IM-9 lymphoblast receptor for substance P.

The neuropeptide substance P (SP) stimulates human T-lymphocyte function in vitro. Human blood T-lymphocytes and cultured human IM-9 B-lymphoblasts express 7,000-10,000 and 25,000-30,000 substance P receptors per cell, respectively. The specific binding of 125I-SP is retained in IM-9 lymphoblast membranes solubilized in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS) at a detergent-to-protein ratio of 1.0. In addition, specific and reversible SP binding to soluble IM-9 cell membrane proteins is demonstrated by gel filtration. The saturation of binding of 125I-SP to both intact and solubilized IM-9 cell membranes attained a steady state after 40-50 min at 4 degrees C. Scatchard analysis of the concentration dependence of 125I-SP binding to IM-9 cell membranes revealed a KD of 0.87 +/- 0.8 nM (mean +/- S.D., n = 4), which is similar to that observed in intact cells, and a density of receptors of 21 +/- 3 fmol/mg of membrane protein (mean +/- S.D.). Binding of 125I-SP to solubilized membranes demonstrated a KD of 0.75 +/- 0.33 nM (mean +/- S.D., n = 3) and a density of receptors of 3.7 +/- 1.5 fmol/mg of membrane protein (mean +/- S.D., n = 3). Affinity cross-linking of 125I-SP by disuccinimidyl suberate to intact IM-9 cells and membranes revealed specifically labeled proteins of Mr 58,000 and 33,000 in cells, and 58,000, 33,000, and 16,000 in membranes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under both reducing and nonreducing conditions. Competitive effects of substituent peptides of SP on cross-linking and 125I-SP binding to membranes demonstrated that the SP receptor recognized the carboxyl-terminal domain of the peptide. Membranes from cells preincubated in vitro for 12 h at 37 degrees C with 10(-8) M SP demonstrated a decrease in SP receptor density to 13 +/- 2 fmol/mg (mean +/- S.D., n = 2), and a parallel diminution in the specific labeling of membrane proteins of Mr 58,000 and 33,000. These observations suggest that solubilization in CHAPS preserves the binding characteristics of the IM-9 lymphoblast receptor for SP, and that affinity cross-linking techniques identify by sodium dodecyl sulfate-polyacrylamide gel electrophoresis membrane proteins that are specifically labeled by SP.