Proteolytic fragments identified with domains of the aspartate chemoreceptor

Two proteolytic fragments generated during the preparation of the aspartate receptor from Salmonella typhimurium have been purified. These fragments are the products of a single cleavage by an endogenous protease after amino acid 259 in the sequence of the intact receptor. Proteolytic fragment 1 (PF1) represents amino acids 1-259 (Mr = 29,000); this unit retains the aspartate-binding function of the intact receptor. The second fragment (PF2) includes residues 260-552 (Mr = 31,000) and has the normal sites of reversible methylation for the receptor. Like the purified intact receptor, this fragment can be methylated in vitro, although at a much slower rate. Circular dichroic measurements suggest that both proteolytic fragments contain substantial alpha-helical structure, approximately 95 and 53% for PF1 and PF2, respectively. No beta-structure could be detected in either fragment. Molecular sieve chromatography in the presence of detergent suggests that PF1 occurs as a stable multimer of an order equivalent to that observed for the detergent-solubilized aspartate receptor, i.e. a tetramer (+/- 1). PF2 is found to have a multimeric form which is sensitive to the removal of detergent. It is proposed that these fragments represent structural and functional domains of the aspartate receptor.