Human neuromodulator SLURP-1: bacterial expression, binding to muscle-type nicotinic acetylcholine receptor, secondary structure, and conformational heterogeneity in solution.

Human protein SLURP-1 is an endogenous neuromodulator belonging to the Ly-6/uPAR family and acting on nicotinic acetylcholine receptors. In the present work, the gene of SLURP-1 was expressed in E. coli. The bacterial systems engineered for SLURP-1 expression as fused with thioredoxin and secretion with leader peptide STII failed in the production of milligram quantities of the protein. The SLURP-1 was produced with high-yield in the form of inclusion bodies, and different methods of the protein refolding were tested. Milligram quantities of recombinant SLURP-1 and its (15)N-labeled analog were obtained. The recombinant SLURP-1 competed with (125)I-α-bungarotoxin for binding to muscle-type Torpedo californica nAChR at micromolar concentrations, indicating a partial overlap in the binding sites for SLURP-1 and α-neurotoxins on the receptor surface. NMR study revealed conformational heterogeneity of SLURP-1 in aqueous solution, which was associated with cis-trans isomerization of the Tyr39-Pro40 peptide bond. The two structural forms of the protein have almost equal population in aqueous solution, and exchange process between them takes place with characteristic time of about 4 ms. Almost complete (1)H and (15)N resonance assignment was obtained for both structural forms of SLURP-1. The secondary structure of SLURP-1 involves two antiparallel β-sheets formed from five β-strands and closely resembles those of three-finger snake neurotoxins.