Purification, crystallization and preliminary crystallographic analysis of the CBS pair of the human metal transporter CNNM4

This work describes the purification and preliminary crystallographic analysis of the CBS‐pair regulatory domain of the human ancient domain protein 4 (ACDP4), also known as CNNM4. ACDP proteins represent the least‐studied members of the eight different types of magnesium transporters that have been identified in mammals to date. In humans the ACDP family includes four members: CNNM1–4. CNNM1 acts as a cytosolic copper chaperone and has been associated with urofacial syndrome, whereas CNNM2 and CNNM4 have been identified as magnesium transporters. Interestingly, mutations in the CNNM4 gene have clinical consequences that are limited to retinal function and biomineralization and are considered to be the cause of Jalili syndrome, which consists of autosomal recessive cone‐rod dystrophy and amelogenesis imperfecta. The truncated protein was overexpressed, purified and crystallized in the orthorhombic space group C 222. The crystals diffracted X‐rays to 3.6 Å resolution using synchrotron radiation. Matthews volume calculations suggested the presence of two molecules in the asymmetric unit, which were likely to correspond to a CBS module of the CBS pair of CNNM4.