Cloning, bacterial expression, and unique structure of adenosylhomocysteine hydrolase-like protein 1, or inositol 1,4,5-triphosphate receptor-binding protein from mouse kidney

Adenosylhomocysteine hydrolase (SAHase)-like protein 1 (SAH-L), also called inositol 1,4,5-triphosphate receptor-binding protein (IRBIT) is a novel protein involved in fish embryo development and calcium release in mammalian cells through protein-protein interactions. To better understand its reaction mechanism, purified protein is indispensable. Here we describe a simple purification procedure and the unique properties of SAH-L. The cDNA was isolated from mouse kidney by RT-PCR and inserted into various pET™ vectors. Escherichia coli harboring a plasmid coding for SAH-L with a C-terminal His-tag could solely produce a soluble protein. SAH-L purified through a Ni²⁺ column gave M_rs of 59,000 and 190,000 by SDS-PAGE and gel filtration, respectively, which is suggestive of a trimer, but chemical cross-linking experiments demonstrated a dimer. The incompatible M_r values implicate an irregular structure of SAH-L. In fact, SAH-L was partially purified in a form lacking the 31 N-terminal residues, and was found to be extremely susceptible to proteases in the region around residue 70. The N-terminal polypeptide (residues 1-98) was also expressed as a soluble form and was trypsin-sensitive. Circular dichroism revealed a low α-helix content but not a randomly extended structure. Interestingly, SAH-L contained tightly bound NAD⁺ despite showing no SAHase activity. The characterized properties of SAH-L and its N-terminal fragment present the notion that the structure of the protease-sensitive N-terminal region is relatively loose and flexible rather than compact, and which protrudes from the major SAHase-like domain. This structure is supposed to be favorable to interact with the IP₃ receptor.