The iodocyanopindolol and SM-11044 binding protein belongs to the TM9SF multispanning membrane protein superfamily

SM-11044 is the only beta -adrenergic agonist that inhibits guinea pig eosi nophil chemotaxis and induces relaxation of depolarized rat colon tonus. We have previously reported the purification of a 34 kDa photo affinity-label ed SM-11044 binding protein (SMBP) from rat colon that may mediate the biol ogical effects of the ligand and that differs from all known monoamine rece ptors (Sugasawa et al., J. Biol. Chem. 272 (1997) 21244). The present repor t describes partial amino acid sequence of rat SMBP and molecular cloning o f corresponding human SMBP (hSMBP) cDNA. This cDNA encodes a 588 amino acid residue polypeptide comprising a signal peptide, a long hydrophilic amino- terminal region, and a highly hydrophobic C-terminal portion organized into nine putative transmembrane domains. The sequence and structure of hSMBP s hows homology to members of a new transmembrane protein 9 superfamily (TM9S F). Comparison of hSMBP with related protein sequences from yeast, plant an d human revealed two subgroups within TM9SF. The members of these groups di ffer in length and have characteristic amino acid sequence motifs in their amino-terminal portion. Northern blot analysis revealed two major SMBP mRNA s, at 3.4 and 3.8 kb, that were present in all the human tissues examined. Western blot experiments detected SMBP as a 70 kDa protein that may be furt her cleaved into an active 34 kDa N-terminal polypeptide. Stable Chinese Ha mster Ovary cell transfectants expressing hSMBP cDNA displayed specific bin ding of [I-125]iodocyanopindolol that was displaced by SM-11044 in a dose-d ependent manner. Thus, SMBP is the first member of TM9SF with functional li -and binding properties, suggesting that some of these integral membrane pr oteins may function as channels, small molecule transporters or receptors. (C) 2001 Published by Elsevier Science B.V.