DEMONSTRATION OF A TANDEM PAIR OF COMPLEMENT PROTEIN MODULES IN GABA(B) RECEPTOR 1A

We have subcloned and expressed the N-terminal portion of the recently sequenced metabotropic GABA receptor, GABA(B)R1a, This region of the receptor contains a complement protein-like amino acid sequence. The p urified 140-residue recombinant protein fragment was soluble and stabl e. Mass spectrometry indicated formation of four disulfide bonds, as e xpected if two complement protein modules (CPs, also known as SCRs, Su shi domains) are formed. The circular dichroism spectrum was unusual a nd characteristic of CPs, Differential scanning calorimetry demonstrat ed a melting point (64 degrees C), and total enthalpy commensurate wit h two fully folded domains. We thus conclude that the la subtype of th e GABAB receptor, but not the Ib subtype, contains a pair of CPs and w e present a three-dimensional model of this region. (C) 1998 Federatio n of European Biochemical Societies.