Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.10.3.1 (tyrosinase)
 9,065 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Tyrosinase is a type I membrane glycoprotein essential for melanin synthesis. Mutations in tyrosinase lead to albinism due, at least in part, to aberrant retention of the protein in the endoplasmic reticulum and subsequent degradation by the cytosolic ubiquitin-proteasomal pathway. A similar premature degradative fate for wild type tyrosinase also occurs in amelanotic melanoma cells. To understand critical cotranslational events, the glycosylation and rate of translation of tyrosinase was studied in normal melanocytes, melanoma cells, an in vitro cell-free system, and semi-permeabilized cells. Site-directed mutagenesis revealed that all seven N-linked consensus sites are utilized in human tyrosinase. However, glycosylation at Asn-290 (Asn-Gly-Thr-Pro) was suppressed, particularly when translation proceeded rapidly, producing a protein doublet with six or seven N-linked core glycans. The inefficient glycosylation of Asn-290, due to the presence of a proximal Pro, was enhanced in melanoma cells possessing 2-3-fold faster (7.7-10.0 amino acids/s) protein translation rates compared with normal melanocytes (3.5 amino acids/s). Slowing the translation rate with the protein synthesis inhibitor cycloheximide increased the glycosylation efficiency in live cells and in the cell-free system. Therefore, the rate of protein translation can regulate the level of tyrosinase N-linked glycosylation, as well as other potential cotranslational maturation events.
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PMID:Translation rate of human tyrosinase determines its N-linked glycosylation level. 1106 24

To gain insight into the role of Oa1, the mouse homolog of the human X-linked ocular albinism 1 protein, its properties and subcellular localization were investigated. Antiserum raised against an expressed segment of the Oa1 protein recognized a band of approximately 48 kDa in immunoblots of extracts of cultured mouse melan-a melanocytes, but not of cells of non-melanocyte origin. When melanocyte extracts were treated with glycopeptidase F, a approximately 44 kDa band appeared. Like the melanogenic enzyme tyrosinase, expression of Oa1 was stimulated by alpha-melanocyte stimulating hormone and inhibited by agouti signal protein. Upon density gradient centrifugation of organelles of melan-a cells, Oa1 protein colocalized with the late endosomal/lysosomal marker Lamp1, but only partial overlap was observed with melanosomal proteins in the high density region of the gradient. Immunofluorescence staining revealed that neither endogenous Oa1 nor an Oa1-green fluorescent protein fusion product colocalized with the melanosomal protein tyrosinase related protein-1 in the cell periphery. In contrast, colocalization of Oa1 and Oa1-green fluorescent protein fusion product with Lamp1 was extensive throughout the cell. These results indicate that Oa1 is a melanocyte-specific integral membrane glycoprotein localized to late endosomes/lysosomes but not mature melanosomes. Considering the microscopic findings in patients with X-linked ocular albinism 1, we speculate that Oa1 may play a role in the trafficking of vesicles to developing melanosomes.
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PMID:The mouse ocular albinism 1 gene product is an endolysosomal protein. 1118 Sep 81

Tyrosinase is a type I membrane glycoprotein whose activity is essential for melanin synthesis. Loss of function mutations in tyrosinase is the cause of oculocutaneous albinism 1. In the milder oculocutaneous albinism 1B form in which mutant proteins retain residual activity, the severity of albinism depends on the type of mutations expressed in the melanocyte. In this study, we show that coexpression of wild-type protein with temperature-sensitive tyrosinase mutants corrects the mutant conformation defect in an activity-dependent manner. Exit from the endoplasmic reticulum and complex carbohydrate processing in the Golgi was promoted when temperature-sensitive tyrosinase mutants were ectopically expressed in host melanocytes carrying wild-type protein even at the nonpermissive temperature. Incubation of transfected melanocytes with DOPA (the cofactor and substrate for tyrosinase), or tyrosine (the substrate), further enhanced processing of ectopic mutant proteins. The analysis of glycosylation-deficient mutants revealed regions in tyrosinase with high, low, and intermediate dependency on glycans for maturation. We concluded that the presence of tyrosinase activity enhances the maturation of temperature-sensitive and glycosylation-deficient forms of tyrosinase. The results may explain the variation in pigmentation and the development of pigment later in life in patients carrying different mutant alleles of oculocutaneous albinism 1B.
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PMID:Coexpression of wild-type tyrosinase enhances maturation of temperature-sensitive tyrosinase mutants. 1219 Aug 74

The maturation of eukaryotic secretory cargo initiates cotranslationally and cotranslocationally as the polypeptide chain emerges into the endoplasmic reticulum lumen. Here, we characterized the cotranslational maturation pathway for the human type I membrane glycoprotein tyrosinase. To recapitulate the cotranslational events, including glycosylation, signal sequence cleavage, chaperone binding, and oxidation, abbreviated transcripts lacking a stop codon were in vitro translated in the presence of semipermeabilized melanocyte membranes. This created a series of ribosome/translocon-arrested chains of increasing lengths, simulating intermediates in the cotranslational folding process. Initially, nascent chains were found to associate with the heat shock protein (Hsp) 70 family member BiP. As the nascent chains elongated and additional glycans were transferred, BiP binding rapidly decreased and the lectin-based chaperone system was recruited in its place. The lectin chaperone calnexin bound to the nascent chain after the addition of two glycans, and calreticulin association followed upon the addition of a third. The glycan-specific oxidoreductase ERp57 was cross-linked to tyrosinase when calnexin and calreticulin were associated. This timing coincided with the formation of disulfide bonds within tyrosinase and the cleavage of its signal sequence. Therefore, tyrosinase maturation initiates cotranslationally with the Hsp70 system and is handed off to the lectin chaperone system that first uses calnexin before calreticulin. Interestingly, divergence in the maturation pathways of wild-type and mutant albino tyrosinase can already be observed for translocon-arrested nascent chains.
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PMID:The cotranslational maturation of the type I membrane glycoprotein tyrosinase: the heat shock protein 70 system hands off to the lectin-based chaperone system. 1595 86

Tyrosinase, a type I membrane glycoprotein, is synthesized and glycosylated in the endoplasmic reticulum (ER) and Golgi. The enzyme is subsequently transported to melanosomes where it participates in melanogenesis. Previous studies showed that the disruption of early ER N-glycan processing by deoxynojirimycin (DNJ), an inhibitor of alpha-glucosidase, suppresses tyrosinase enzymatic activity and melanogenesis. However, the disruption of late glycan processing, mainly performed by ER and Golgi alpha-1,2-mannosidases, on tyrosinase enzymatic activity and melanogenesis remains to be investigated. Following treatment of HM3KO human melanoma cells with deoxymannojirimycin (DMJ), an inhibitor of alpha-1,2-mannosidase, transport of tyrosinase to the melanosome, enzymatic activity, and melanogenesis were reduced in a dose-dependent manner. However, DMJ did not directly inhibit tyrosinase enzymatic activity and expression. Interestingly, an extract of Streptomyces subrutilus culture medium (ESSCM) containing DMJ and DNJ as the main components inhibited glycosylation and transport of tyrosinase to the melanosome as well as melanin synthesis, but with no negative effects on cell viability. These inhibitory effects of ESSCM were stronger than those of DMJ or DNJ alone. Tyrosinase glycosylation and melanogenesis in HM3KO melanoma cells were more effectively inhibited by DMJ and DNJ combined than DMJ or DNJ alone. Accordingly, we propose that ESSCM is a potential candidate for treating undesirable hyperpigmentation conditions, such as melasma, postinflammatory melanoderma, and solar lentigo.
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PMID:Influence of N-glycan processing disruption on tyrosinase and melanin synthesis in HM3KO melanoma cells. 1722 24

Human tyrosinase (Tyr) is a Type I membrane glycoprotein that is the rate-limiting enzyme for controlling the production of melanin pigment in melanosomes. Currently, ~300 Tyr mutations are known to be involved in the genetic disease oculocutaneous albinism type 1 (OCA1), which exists in two forms, OCA1A and OCA1B. OCA1A is caused by a full loss of Tyr enzymatic activity, resulting in the absence of pigment in the skin, hair, and eyes, while OCA1B has reduced Tyr activity and pigment. Here, we used molecular modeling to try to understand the role of genetic changes at the protein level in inherited disease. The significant part of Tyr intra-melanosomal domain and five OCA1 mutant variants were built by homology modeling, glycosylated in silico, and refined using molecular dynamics in water. The modeling confirmed experimental results that N347 and N371 glycosylation is vital for protein stability. The changes caused by the T373K mutation indicate a significant impact on protein structure, as expected for OCA1A. In addition, evaluation of free energy changes in OCA1B mutants showed a strong association with the changes observed in our unfolding/refolding experiments in vitro. In conclusion, our results could be useful for understanding the role of OCA1 mutant variants in melanin pigment production, in silico searching for inhibitors and activators of tyrosinase activity, and genotype-to- phenotype analysis in OCA1.
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PMID:Dynamic analysis of human tyrosinase intra-melanosomal domain and mutant variants to further understand oculocutaneous albinism type 1. 3086 38