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J Biol Chem, Vol. 273, Issue 48, 31801-31805, November 27, 1998
From the Mammalian melanogenesis is regulated directly or
indirectly by over 85 distinct loci. The
Tyr/albino locus, in which mutations cause a
lack of pigmentation, encodes tyrosinase (Tyr), the critical and
rate-limiting melanogenic enzyme. Other melanogenic enzymes include
Tyrp1 (or TRP1) and 3,4-dihydroxyphenylalanine-chrome tautomerase (Dct
or TRP2) encoded at the Tyrp1/brown and
Dct/slaty loci, respectively. Murine Tyrp1 can
oxidize 5,6-dihydroxyindole-2-carboxylic acid (DHICA) produced by Dct,
but mutations in Tyrp1 also affect the catalytic functions of Tyr. All
three enzymes are membrane-bound melanosomal proteins with similar
structural features and are thought to interact within and stabilize a
melanogenic complex. We have now further investigated the effect of a
Tyrp1b mutation on Tyr stability. Pulse/chase
labeling experiments show that Tyr is degraded more quickly in
Tyrp1b mutant melanocytes than in melanocytes
wild type at that locus. This reduced stability of Tyr can be partly
rescued by infection with the wild type Tyrp1 gene, and
this is accompanied by phenotypic rescue of infected melanocytes. In
sum, these results suggest that, in addition to its catalytic function
in oxidizing DHICA, Tyrp1 may play an important role in stabilizing
Tyr, a second potential role in the regulation of melanin formation.
Tyrosinase-related protein 1 (Tyrp1, also known as TRP1 and gp75)
is encoded by the Tyrp1/brown locus, one of more
than 85 genes that directly or indirectly affect coat color in mice
(1-4). Tyrp1 is expressed specifically in melanocytes and functions in melanin synthesis within melanosomes, as do the other members of the
tyrosinase-related protein
(TRP)1 family, which includes
Tyr (also known as tyrosinase), and Dct (also known as DOPAchrome
tautomerase and TRP2). Although TRPs have many similar structural
features, including a transmembrane region, two metal-binding regions,
and a cysteine-rich epidermal growth factor motif, each member of the
TRP family has a distinct catalytic activity in the biosynthesis of the
melanin biopolymer. Tyr, encoded by the
Tyr/albino locus, catalyzes the critical and rate-limiting step of tyrosine hydroxylation to
3,4-dihydroxyphenylalanine (DOPA), DOPA oxidation to DOPAquinone, and
5,6-dihydroxyindole (DHI) oxidation to indole-5,6-quinone (5-7). Dct,
encoded by the Dct/slaty locus, functions as
DOPAchrome tautomerase, producing DHI-2-carboxylic acid (DHICA) from
DOPAchrome rather than the spontaneously decarboxylated product DHI (8,
9). Although Tyrp1 was the first member of the TRP family to be cloned
(10), its specific function in melanogenesis has been controversial (11-15). Recently we have shown that murine Tyrp1 functions as a DHICA
oxidase in the melanogenic pathway (16, 17). This catalytic activity of
Tyrp1 promotes the oxidation and polymerization of DHICA monomers into
melanin, and in fact, melanins in the hair of brown mice are
significantly less polymerized than those in black mice (18, 19). There
are other distinct differences in DHI- and DHICA-derived melanins,
including their color, size, and solubility.
Tyr, Tyrp1, and Dct are localized in the melanosomal membrane and act
within the context of a series of reactions in the melanogenic pathway.
It has been proposed that TRPs might interact in a multi-enzyme complex
(or metabolon), because each of them contains a cysteine-rich epidermal
growth factor motif thought to be involved in protein-protein interactions (8). Orlow and co-workers (20) have shown that Tyr, Tyrp1,
and Dct co-purify from murine melanoma cells in high molecular weight
aggregates, and our group has demonstrated that Tyr activity in
vitro is more stable in the presence of Tyrp1 and/or Dct (21, 22).
Thus we suggested that TRPs might interact in vivo and be
stabilized in a multi-enzyme complex, which plays an important role in
the regulation of mammalian melanogenesis. Because Tyrp1 is the most
abundant glycoprotein expressed in melanocytes (23, 24), it may play a
key role in forming that multi-enzyme complex.
There are various murine mutations at the Tyrp1 locus
(e.g. Tyrp1lt/light or
Tyrp1c/cordovan) with known molecular
lesions (25, 26). Well known among them is the
Tyrp1b/brown mutation, which results
in the substitution of tyrosine for cysteine at position 86 (C86Y)
within the epidermal growth factor domain thought to function in
protein-protein interactions. This mutation results in the
mislocalization of Tyrp1, which is not delivered to melanosomes
correctly (27).
To further characterize the interaction of Tyrp1 and Tyr within
melanocytes, we have investigated the effect of the
Tyrp1b mutation on the stability of Tyr in
vivo. We used the murine melanocyte line melan-b, which is
homozygous for the Tyrp1b mutation, and the
melan-a melanocyte line, wild type at this locus (28, 29). In this
study, we compared the stability of Tyr in melan-b cells and in melan-a
cells. Pulse labeling and chase experiments showed that Tyr is degraded
more quickly in Tyrp1 mutant melan-b cells than in the wild type
melan-a cells. Moreover, the lesser stability of Tyr in melan-b cells
can be partly rescued by infection with the wild type Tyrp1
gene, along with phenotypic rescue of those cells. These results
demonstrate that in addition to its catalytic function in DHICA
oxidation, Tyrp1 also plays an important role in the stabilization of
Tyr in melanocytes and might thus also indirectly contribute to the regulation of melanin production.
Cells and Culture Conditions--
Melanocyte lines cultured from
black (Tyrp1/Tyrp1) and from brown
(Tyrp1b/Tyrp1b) nonagouti
mice (melan-a and melan-b cells, respectively) were cultured as
previously reported (28, 29). NIH3T3 murine fibroblasts were cultured
in Dulbecco's modified Eagle's medium containing 10% fetal bovine
serum, 100 units/ml penicillin, and 100 µg/ml streptomycin.
Antibodies Used--
The peptide antibodies used in this study,
Metabolic Labeling and Immunoprecipitation--
These techniques
were performed as reported previously (9, 12, 13, 17, 30, 31). Briefly,
semiconfluent cells were preincubated in methionine-free medium
containing dialyzed fetal bovine serum, pulsed for 30 min with
[35S]methionine (0.1-1.0 mCi/ml) (NEN Life Science
Products) in methionine-free medium, and chased in complete medium for
0-48 h, as detailed in the figure legends. The cells were washed with
phosphate-buffered saline without Mg2+ and Ca2+
and solubilized at 4 °C for 60 min in Nonidet P-40/SDS buffer (1%
Nonidet P-40, 0.01% SDS, 0.1 M Tris-HCl, pH 7.2, 100 µM phenylmethylsulfonyl fluoride, 1 µg/ml aprotinin).
The extracts were centrifuged for 15 min at 10,000 × g, and the supernatants were kept at 4 °C for immunoprecipitation. The resulting 35S-labeled extracts
were then precleared with 10 µl of normal rabbit serum and 100 µl
of protein G-Sepharose 4FF (Amersham Pharmacia Biotech). Two × 106 trichloracetic acid-precipitable cpm of precleared
extracts were incubated with 5 µl of antibodies for 1 h at
4 °C and were then complexed with 30 µl of protein G-Sepharose for
30 min at 4 °C. The immune complexes were washed six times with
Nonidet P-40/SDS buffer, eluted in SDS sample buffer containing
2-mercaptoethanol at 95 °C for 3 min and analyzed by SDS gel
electrophoresis (32), followed by fluorography. For immunoprecipitation
using Infection of the Wild Type Tyrp1 Gene into Melan-b
Melanocytes--
Retroviral infection of melan-b cells was performed
as described previously (33). Briefly, psi-2 helper fibroblasts
transfected with the Tyrp1 retrovirus (pHS-TRP1) and with pSV2-neo were
selected in G418 (200 µg/ml) and were incubated for 12 h in
melanocyte medium without 12-O-tetradecanoylphorbol
13-acetate. The conditioned medium containing virus was harvested,
filtered through a 22-µm membrane, supplemented with 200 nM 12-O-tetradecanoylphorbol 13-acetate and 10 µg/ml polybrene, and placed in melan-b cultures plated the previous
day at 2 × 104 cells/ml. The medium was replaced
after 12 h with freshly prepared conditioned medium. After three
such exposures to the conditioned medium, cells were cultured in fresh
melanocyte medium for 2 days and were then subcultured and subcloned.
Southern Blotting Analysis--
Southern blotting was performed
with some modification as described previously (33). Briefly, DNA was
isolated from melanocytes with a Qiagen DNA isolation kit, according to
the manufacturer's instructions. SacI-digested DNA was
separated by agarose gel electrophoresis, transferred to a Hybond-N
membrane (Amersham Pharmacia Biotech) and hybridized with a
fluorescein-labeled HindIII-digested pMT4 probe for the
murine Tyrp1 gene (10, 34), followed by visualization with a
Fluorescein Gene Image detection system (Amersham Pharmacia Biotech)
according to the manufacturer's instructions.
Quantification of Pulse-labeled Tyr by Immunoprecipitation with
Degradation of Tyr in Melan-a and Melan-b Melanocytes--
The
synthesis and degradation of melanogenic proteins in melan-b
(Tyrp1b/Tyrp1b) and
melan-a (Tyrp1/Tyrp1) melanocytes was compared
using peptide antibodies specific for each protein (Fig.
2A). As found previously (9,
12, 17), two Tyr bands from melan-a melanocytes were observed, a higher
broad band and a lower band, representing glycosylated and de
novo Tyr, respectively. In melan-a and in melan-b melanocytes, Dct
and Pmel17 (also known as gp100 and the silver protein) were expressed
and degraded similarly. In contrast, Tyrp1 in mutant melan-b cells was
not glycosylated to a significant degree and disappeared more quickly
than in wild type melan-a melanocytes. Tyr was also underglycosylated
to some extent and was degraded significantly more quickly in melan-b
cells than in melan-a cells, even though both types of cells produce
genetically identical Tyr. Densitometric analysis of three independent
pulse/chase experiments showed clearly that Tyr and Tyrp1 in melan-b
cells were degraded significantly more quickly than in melan-a cells,
whereas rates of degradation of Dct and Pmel17 were virtually identical
(Fig. 2B). As the rate of processing of Tyr from the
de novo type (i.e. the lower molecular weight
band) to the highly glycosylated type (i.e. the higher
molecular weight band) was decreased in melan-b cells compared with
melan-a cells, we analyzed the degradation of the glycosylated Tyr band
in melan-a and melan-b cells (Fig. 2C). The amount of
glycosylated Tyr present at the 1.5-h chase period was adjusted to
100% as a control, and it is clear that even glycosylated Tyr was
degraded significantly more quickly in melan-b cells than in melan-a
cells.
Tyr is inserted and anchored in the melanosomal membrane via its
transmembrane region (near the carboxyl-terminal) and can be readily
digested proteolytically there to produce a smaller Tyr with a
truncated carboxyl terminus but which still retains enzymatic activity
(12, 35). Therefore, it was possible that only the carboxyl terminus of
Tyr might have been lost during the pulse/chase experiments reported
above, which might be misleading when using only the Tyr Stability in Melan-b Melanocytes Infected with Wild Type
Tyrp1--
We next compared the degradation of Tyr in uninfected and
in Tyrp1-infected melan-b melanocytes. When melan-b cells
were infected with the wild type Tyrp1 gene and cultured for
10 days, some black cells (i.e. phenotypically rescued
cells) were observed among the infected cell population, as reported
previously (33). Such cells represented about 20% of the total
infected melan-b cell population. We subcloned these cells by limited
dilution cloning in 96-well plates and established several cloned
lines. Clones 1 and 2 were produced from mock-infected melan-b cells,
whereas clones 3, 4, and 5 were from Tyrp1-infected melan-b
cells. The relatively long term culture of cloned
Tyrp1-infected cell lines without selective pressure caused
the emergence of a heterogeneous population, including some cells that
appeared brown again. Southern blotting analysis (Fig.
4, inset) demonstrated that
uninfected clones 1 and 2 did not contain wild type Tyrp1,
as expected, whereas clones 3, 4, and 5 contained the infected wild
type Tyrp1 minigene. This is consistent with the phenotypes
of their cell pellets, i.e. pellets of clones 1 and 2 were
brown, whereas pellets of clones 3, 4, and 5 were black. When
degradation of Tyr in these subcloned cells was analyzed with
pulse/chase labeling, Tyr was significantly more stable following
infection of melan-b melanocytes with the wild type
Tyrp1 gene (Fig. 4).
This study demonstrates that Tyr (encoded at the
Tyr/albino locus) is degraded more quickly in
melan-b melanocytes, which are homozygous for the brown mutation
(Tyrp1b/Tyrp1b), than in
melan-a melanocytes, wild type at that locus. Moreover, the reduced
stability of Tyr and the brown phenotype of melan-b melanocytes could
be rescued by infection with the wild type Tyrp1 gene. Thus,
the decreased stability of Tyr in melan-b cells is caused by the C86Y
point mutation in the Tyrp1 gene. Our previous study (22)
showed clearly that Tyr catalytic function in melan-b cells is only
~65% that found in melan-a cells, whereas Dct function (as
DOPAchrome tautomerase) is identical in both cell lines. Several clues
about the important role of Tyrp1 in Tyr function have also been
provided by in vivo studies. Tyr catalytic function in the skin of brown mice is only 80% that found in the skin of black mice
(36), and melanin content of brown mouse hair is only 30-35% that
found in black mouse hair (19, 36).
Because the only difference between these black and brown melanocytes
(and animals) is the point mutation in the Tyrp1 gene, how
can the instability of Tyr in Tyrp1-mutant melanocytes be explained?
One attractive possibility is that the brown mutation affects the
direct interaction between Tyr and Tyrp1 and suppresses the stabilizing
influence of Tyrp1 on Tyr. Current studies by our group and others have
demonstrated that Tyr and Tyrp1 are localized in a melanosomal enzyme
complex and interact with each other (20, 21). The brown mutation
(C86Y) results in a structural change in the epidermal growth factor
motif of Tyrp1, a domain thought to play an important role in
protein-protein interactions (25). The mutation appears to allow the
rapid degradation of under-glycosylated Tyrp1, resulting in the absence
of Tyrp1 in melanosomes (27), which would in turn reduce direct
interaction of Tyrp1 with Tyr. Decreases in the glycosylation of Tyrp1
and Tyr suggest that these two proteins might interact in some manner in the Golgi apparatus where such glycosylation occurs. The altered glycosylation might affect proper routing of these proteins to the
melanosome and might thus also be an important factor in their rapid degradation.
Mutations in Tyrp1 also lead to structural changes in melanosomes
(36-40), which might, at least in part, result from the reduced stability of Tyr and Tyrp1 observed in this study. Melanosomes produced
in brown melanocytes (i.e.
Tyrp1b/Tyrp1b) are round,
particulate, and relatively disorganized in structure, rather than
ovoid, lamellar, and regular structures as normally observed on a
eumelanogenic background. Further, other mutations at the
Tyrp1 locus (e.g. Tyrp1lt)
cause the premature death of follicular melanocytes coordinated with
melanogenesis, suggesting that Tyrp1 functions in some manner to
protect cells from toxic intermediates during melanin biosynthesis (26). One possible mechanism for Tyr degradation within the melanosome
might be its susceptibility to attack and cleavage by melanin
intermediates (such as quinones) in melanosomes of Tyrp1 dysfunctional melanocytes.
Infection of the wild type Tyrp1 gene into melan-b melanocytes resulted
in only a partial rescue of Tyr instability. Even in subcloned
Tyrp1-infected cells, Tyrp1 was not fully glycosylated and disappeared
relatively quickly, as it did in uninfected melan-b cells (data not
shown). Incomplete rescue of Tyr stability in melan-b melanocytes by
Tyrp1 gene infection might result from insufficient Tyrp1
expression and/or function to fully stabilize Tyr as it does in wild
type melan-a cells. A further complication is that reversion to the
parental brown phenotype occurs rather quickly, which may result from
methylation of viral DNA (33). It seems likely that, were it possible
to analyze the stability of Tyr solely in cells expressing infected
wild type Tyrp1, rescue of Tyr stability would be significantly more dramatic.
It is not completely clear how the decreased stability of Tyr in Tyrp1
mutant melanocytes contributes to the brown phenotype. The decrease in
Tyr activity necessarily results in a decrease in melanin production,
as has been shown for human and mouse brown hair. Failure of routing of
Tyrp1 to melanosomes in Tyrp1 mutant melanocytes must eliminate Tyrp1
function(s) in melanosomes, including its catalytic activity. In
humans, one type of human TYR-positive albinism, termed OCA3, has been
identified recently that maps at the human homolog of the
Tyrp1//brown locus (41). Melanocytes isolated
from that OCA3 patient have no TYRP1 expression, and although the
transcription and translation of TYR is detected at normal levels,
melanin production is diminished drastically, similar to what is
observed in melan-b melanocytes. Human TYRP1 does not seem to have
DHICA oxidase activity (42), and thus the TYR stabilizing function of
TYRP1 may in general be even more important to the regulation of
mammalian pigmentation than is its catalytic function. Human TYR is
significantly less stable than murine Tyr
(7),2 and the stabilizing
role of Tyrp1 on Tyr may play a more active and essential role in the
regulation of human pigmentation than in mouse pigmentation. Further
studies on TYR stability in OCA3-derived and normal human melanocytes
will be necessary to clarify this point. In sum, these results suggest
that in addition to its catalytic function(s), the principal function
of Tyrp1 may be to stabilize Tyr in melanocytes, and Tyrp1 might thus
regulate basal levels of melanin formation.
*
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
The abbreviations used are:
TRP, tyrosinase-related protein; DHI, 5,6-dihydroxyindole; DHICA, DHI-2-carboxylic acid; DOPA, 3,4-dihydroxyphenylalanine; Tyr, tyrosinase; Dct, DOPAchrome tautomerase.
2
T. Kobayashi, G. Imokawa, D. C. Bennett,
and V. J. Hearing, unpublished results.
Tyrosinase Stabilization by Tyrp1 (the brown Locus
Protein)*
§,
Laboratory of Cell Biology, NCI, National
Institutes of Health, Bethesda Maryland 20892, the
§ Biological Science Laboratories, Kao Corporation, Tochigi
321-34 Japan, and the ¶ Department of Anatomy, St. George's
Hospital Medical School, London SW17 0RE, United Kingdom
ABSTRACT
Top
Abstract
Introduction
Procedures
Results
Discussion
References
INTRODUCTION
Top
Abstract
Introduction
Procedures
Results
Discussion
References
EXPERIMENTAL PROCEDURES
Top
Abstract
Introduction
Procedures
Results
Discussion
References
PEP1, PEP7, PEP8, and PEP13, were generated in rabbits
against synthetic peptides corresponding to the carboxyl termini of
murine melanogenic proteins Tyrp1, Tyr, Dct, and Pmel17 (also known as
the silver protein), respectively, and specifically recognize each
melanogenic protein as described previously (9, 12, 30). Another
antibody used in this study, PEP5, was produced against the
amino-terminal peptide of murine Tyr, an epitope recognized only
following denaturation, as detailed previously (12).
PEP5 antibody, labeled cell extracts were denatured before
incubation with antibody as reported previously (12). For
quantification of immunoprecipitated bands, fluorographs were analyzed
with BioImage (Millipore).
RESULTS
Top
Abstract
Introduction
Procedures
Results
Discussion
References
PEP7--
To quantify the degradation of Tyr within melanocytes, we
used pulse/chase metabolic labeling. Tyr in the metabolically labeled melan-a melanocyte extract was specifically immunoprecipitated with
PEP7 antibody, separated by SDS gel electrophoresis, visualized by
fluorography, and densitometrically analyzed (Fig.
1). An excellent correlation was obtained
between the relative intensity of the immunoprecipitated Tyr and the
amount of melanocyte cell extract added, showing that the degradation
of Tyr can be quantified using such immunoprecipitation analysis.
View larger version (29K):
[in a new window]
Fig. 1.
Quantification of immunoprecipitated Tyr by
fluorography. The correlation between the amount of radiolabeled
melanocyte extract used for immunoprecipitation and the intensity of
the immunoprecipitated Tyr band detected by fluorography was analyzed.
Melan-a melanocytes and NIH 3T3 fibroblasts were metabolically labeled
with [35S]methionine for 4 h, solubilized in Nonidet
P-40/SDS buffer and precleared with normal rabbit serum and protein
G-Sepharose, as detailed under "Experimental Procedures." The
specified acid-precipitable cpm in precleared melan-a extracts (× 10 
3, inset) were combined with cpm in precleared extracts
of NIH 3T3 cells to make a total of 107 cpm in each sample
and then were immunoprecipitated with PEP7 and analyzed by
fluorography (inset) followed by densitometry. The
line drawn and the noted coefficient of determination were
calculated by the least squares method from quadruplicate assays for
31,600-3,160,000 cpm of melan-a extract.
View larger version (35K):
[in a new window]
Fig. 2.
Degradation of melanogenic proteins in
melan-a and melan-b melanocytes. Melan-a and melan-b melanocytes
were metabolically labeled with [35S]methionine for 30 min and chased in complete medium for 1.5-48 h, as noted. Two
×106 acid-precipitable counts of labeled cell extracts
were subjected to immunoprecipitation with antibodies as listed, and
immunoprecipitated melanogenic proteins were analyzed by gel
electrophoresis and fluorography, as detailed under "Experimental
Procedures." A, autoradiographs of immunoprecipitated
bands. B, densitometric plots of bands (data from three
independent experiments). Analysis of variance shows that curves for
Tyr and Tyrp1 differed significantly (p < 0.05)
between melan-a and melan-b, whereas curves for Dct and Pmel17 were not
significantly different. Data from three independent experiments are
reported as means ± S.E. *, p < 0.05 at specific
time points. C, densitometric plot of glycosylated Tyr
stability (adjusted to 100% at 1.5 h). Analysis of variance shows
that these curves differed significantly (p < 0.01)
PEP7 antibody
(which recognizes that carboxyl terminus). Therefore, we also
immunoprecipitated Tyr using PEP5 (which recognizes the amino
terminus of Tyr) and analyzed its degradation in melan-a and melan-b
cells. Again, a decrease in glycosylation and a significantly lowered
stability of Tyr in melan-b cells was observed (Fig.
3), indicating that the loss of
immunoreactivity represents degradation of the entire protein and not
just its carboxyl terminus.
View larger version (39K):
[in a new window]
Fig. 3.
Stability of Tyr measured using PEP5
antibody. Melan-a and melan-b melanocytes were pulse/chase labeled
as described for Fig. 2 and then immunoprecipitated with PEP5
antibody, which recognizes the amino terminus of Tyr. A,
autoradiographs of immunoprecipitated Tyr bands. B,
densitometric plots of bands shown in A. C,
densitometric plots of glycosylated Tyr band shown in A,
adjusted to 100% at 1.5 h (similar analysis as shown in Fig. 2,
panels B and C). Statistical analysis of variance
shows that the curves in each panel differ significantly at
p < 0.02.
View larger version (36K):
[in a new window]
Fig. 4.
Tyr stability in uninfected and
Tyrp1-infected melan-b melanocytes. The stability of glycosylated
Tyr in subclones of uninfected melan-b melanocytes (clones 1 and 2) and
melan-b melanocytes infected with wild type Tyrp1 (clones 3, 4, and 5) were measured as detailed for Fig. 2C.
Inset, Southern blot analysis of 2 µg of genomic DNA
isolated from subcloned cells. DNA was cut with SacI and
subjected to Southern blot hybridization, using a fluorescein-labeled
HindIII-digested probe of pMT4. Analysis of variance
statistical analysis shows that curves for the combined data for clones
1 and 2 (uninfected) differ significantly from that of clones 3, 4, and
5 (Tyrp1 infected) at p < 0.01.
DISCUSSION
Top
Abstract
Introduction
Procedures
Results
Discussion
References
FOOTNOTES
To whom correspondence should be addressed: Lab. of Cell
Biology, Bldg. 37, Rm. 1B25, NIH, Bethesda, MD 20892. Tel.:
301-496-1564; Fax: 301-402-8787; E-mail: hearingv{at}nih.gov.
REFERENCES
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Abstract
Introduction
Procedures
Results
Discussion
References
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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 M. Tsatmali, J. Ancans, and A. J. Thody Melanocyte Function and Its Control by Melanocortin Peptides J. Histochem. Cytochem., February 1, 2002; 50(2): 125 - 134. [Abstract] [Full Text] [PDF]
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 K. TOYOFUKU, I. WADA, J. C. VALENCIA, T. KUSHIMOTO, V. J. FERRANS, and V. J. HEARING Oculocutaneous albinism types 1 and 3 are ER retention diseases: mutation of tyrosinase or Tyrp1 can affect the processing of both mutant and wild-type proteins FASEB J, October 1, 2001; 15(12): 2149 - 2161. [Abstract] [Full Text] [PDF]
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 R. Halaban, S. Svedine, E. Cheng, Y. Smicun, R. Aron, and D. N. Hebert Endoplasmic reticulum retention is a common defect associated with tyrosinase-negative albinism PNAS, May 23, 2000; 97(11): 5889 - 5894. [Abstract] [Full Text] [PDF]
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J. F. Berson, D. W. Frank, P. A. Calvo, B. M. Bieler, and M. S. Marks A Common Temperature-sensitive Allelic Form of Human Tyrosinase Is Retained in the Endoplasmic Reticulum at the Nonpermissive Temperature J. Biol. Chem., April 14, 2000; 275(16): 12281 - 12289. [Abstract] [Full Text] [PDF]
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G. Imokawa, T. Kobayasi, and M. Miyagishi Intracellular Signaling Mechanisms Leading to Synergistic Effects of Endothelin-1 and Stem Cell Factor on Proliferation of Cultured Human Melanocytes. CROSS-TALK VIA TRANS-ACTIVATION OF THE TYROSINE KINASE c-KIT RECEPTOR J. Biol. Chem., October 20, 2000; 275(43): 33321 - 33328. [Abstract] [Full Text] [PDF]
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G. Negroiu, R. A. Dwek, and S. M. Petrescu Folding and Maturation of Tyrosinase-related Protein-1 Are Regulated by the Post-translational Formation of Disulfide Bonds and by N-Glycan Processing J. Biol. Chem., October 6, 2000; 275(41): 32200 - 32207. [Abstract] [Full Text] [PDF]
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