CRYM

Mu-crystallin homolog also known as NADP-regulated thyroid-hormone-binding protein (THBP) is a protein that in humans is encoded by the CRYM gene. Multiple alternatively spliced transcript variants have been found for this gene.^[5][6]

CRYM
Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 2I99
Identifiers
Aliases	CRYM, DFNA40, THBP, crystallin mu
External IDs	OMIM: 123740; MGI: 102675; HomoloGene: 1424; GeneCards: CRYM; OMA:CRYM - orthologs
Gene location (Human) Chr. Chromosome 16 (human)[1] Band 16p12.2 Start 21,238,874 bp[1] End 21,303,083 bp[1]
Gene location (Mouse) Chr. Chromosome 7 (mouse)[2] Band 7 F2|7 64.47 cM Start 119,785,603 bp[2] End 119,801,334 bp[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in cerebellar vermis frontal pole paraflocculus of cerebellum right hemisphere of cerebellum Brodmann area 10 orbitofrontal cortex apex of heart nucleus accumbens Brodmann area 9 right frontal lobe Top expressed in dentate gyrus of hippocampal formation granule cell hippocampus proper olfactory tubercle nucleus accumbens subiculum lip dorsal striatum Region I of hippocampus proper superior frontal gyrus CA3 field More reference expression data BioGPS More reference expression data
Gene ontology Molecular function oxidoreductase activity hormone binding protein binding protein homodimerization activity transcription corepressor activity NADP binding thyroid hormone binding thiomorpholine-carboxylate dehydrogenase activity oxidoreductase activity, acting on the CH-NH2 group of donors, NAD or NADP as acceptor Cellular component cytoplasm extracellular exosome mitochondrion nucleus peroxisomal matrix cytosol Biological process thyroid hormone transport negative regulation of transcription by RNA polymerase II hearing lysine catabolic process thyroid hormone metabolic process Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 1428 12971 Ensembl ENSG00000103316 ENSMUSG00000030905 UniProt Q14894 O54983 RefSeq (mRNA) NM_001014444 NM_001888 NM_001376256 NM_016669 RefSeq (protein) NP_001879 NP_001363185 NP_057878 Location (UCSC) Chr 16: 21.24 – 21.3 Mb Chr 7: 119.79 – 119.8 Mb PubMed search [3] [4]
Wikidata
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Function

Crystallins are separated into two classes: taxon-specific and ubiquitous. The former class is also called phylogenetically-restricted crystallins. The latter class constitutes the major proteins of vertebrate eye lens and maintains the transparency and refractive index of the lens. This gene encodes a taxon-specific crystallin protein that binds NADPH and has sequence similarity to bacterial ornithine cyclodeaminases. The encoded protein does not perform a structural role in lens tissue, and instead it binds thyroid hormone for possible regulatory or developmental roles.^[6]

Its enzyme function has been determined as a ketimine reductase, reducing cyclic ketimines to their reduced forms. Either NADH or NADPH can be used as cofactor. The most active substrate at pH 5.0 is aminoethylcysteine ketimine (AECK), however at neutral pH (pH 7.2) the most active substrate is 1-piperideine-2-carboxylate which is an important part of the pipecolic acid pathway. The active form of thyroxine, T₃, has been found to be a potent inhibitor at nanomolar concentrations.^[7]

Besides its role in lens biology, CRYM seems also to be involved in thyroid hormone signalling in other tissues. It could be demonstrated that CRYM mutations may cause deafness through thyroid hormone binding effects on the fibrocytes of the cochlea.^[8] Disruption of the CRYM gene leads to decreased T₃ concentrations in both tissues and serum without alteration of peripheral T₃ action in vivo.^[9][10]

The existence of intracellular thyroid hormone binding proteins has been postulated from mathematical modelling of pituitary-thyroid homeostasis.^[11] Binding properties have been assumed to be similar to those of extracellular binding proteins,^[12] however it is not clear, if THBP is the only intracellular thyroid hormone binding protein.