The CRD2 is a complex protein that contains seven CXXC metal-binding domains, and is thought to be important for copper homeostasis in eukaryotic cells. The human gene for CRD2 is located on chromosome 11p11.1. A number of other proteins with similar function have been identified in prokaryotic genomes, including copper transporters and oxidases. These prokaryotic homologs of the Menkes and Wilson disease proteins have multiple CXXC motifs in their N-terminal regions, but none contain as many as the Crd1p protein does.
Crd1p is involved in copper export and transport between the cytoplasm and the extracellular matrix, thereby controlling copper uptake in the yeast cell (25). Like other metallotransporters, the CRD2 gene encodes a protein that contains six CXXC copper-binding domains in its N-terminal region, and five of these contain Cys-Xaa-Cys repeats. These domains are believed to coordinate copper binding, but they differ from the copper-binding sites of the copper oxidases and transporters in that they do not bind reversibly or catalytically.
Unlike most other Cu-transporting ATPases, the copper export domain of Crd1p also has an acidic domain that may play a role in determining substrate specificity. A similar acidic domain is present in the cytoplasmic copper-binding protein CopA of Enterobacter hirae, which is required for export of the copper-dependent enzyme Fet3p in this organism.
An alanine to lysine substitution at the hexapeptide linker of the helix-hairpin-like coiled structure of Crd1p is known to alter its substrate recognition and affinity. The alanine to lysine mutation decreases the affinity of Crd1p for the copper atom, but does not affect the protein's ability to transport copper from the cytoplasm to the extracellular matrix.
Analysis of CRD1 and Crd2 expression in response to metals showed that transcription of both genes increased with increasing levels of copper in the growth medium. However, when the Crd1 gene was disrupted, the strain became sensitive to copper, and failed to grow in media containing 100 mM of CuSO4. The transcription of the Crd2 gene is not affected by the presence of metals in the growth medium.
When plasma from the same cats was used to neutralise pseudotypes bearing 21 Env variants, it was found that samples collected from sick cats (displaying clinical symptoms) harboured a significantly greater proportion of CRD2-independent Env variants than did plasma from healthy cats (p = 0.75, Fisher's exact test). This suggests that key factors driving Env evolution are linked to changes in host status.
Both CRD genes are essential for normal vision, but the two phenotypes caused by mutations in this gene are independent of each other, and crossing carriers of both alleles will not produce affected offspring. The CRD2 mutation is recessive, so a dog with two copies of the normal allele must have one copy of the CRD2 allele in order to be affected by the disorder. In contrast, the CRD1 allele is dominant over the non-mutant allele. This makes the disorder more common in dogs. In addition, the prevalence of the disorder varies with the breed of dog: the German shepherd is the most commonly affected breed of dog, but other large breeds such as terriers, poodles, and cocker spaniels are also susceptible.
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