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Melanosomal tyrosine transport in normal and pink-eyed dilution murine melanocytes
W A Gahl1, B Potterf, D Durham-Pierre
1Section on Human Biochemical Genetics, Human Genetics Branch, NICHD, NIH, Bethesda, Maryland 20892-1830, USA.
Abstract:
Tyrosine is the endogenous substrate for melanin production within melanosomes, but the method of tyrosine transport into the melanosome has not been investigated. In the mouse, melanogenesis is disrupted by mutations in the p gene resulting in the pink-eyed dilution phenotype; it has been suggested that the p gene codes for a tyrosine transport protein. We determined that normal (melan-a) melanosome-rich granular fractions take up 10 microns [3H]tyrosine at 21.1 +/- 6.1 (SEM, standard error of the mean) pmol/min/mg protein (N = 7) compared with 21.3 +/- 5.8 SEM pmol/min/mg protein (N = 5) for pink-eyed dilution, whose plasma membrane tyrosine transport was also normal (Km 89 microM; Vmax 302 pmol/min/mg cell protein). We also demonstrated that pink-eyed dilution melanosomes are immature by virtue of their low density, high hexosaminidase activity, and lack of pigment. These data indicate that a tyrosine transport system exists in the melanosomal membrane and that the p gene does not encode a tyrosine transporter of critical importance.
Insights
Melanogenesis relies on tyrosine transport into melanosomes. This study found no critical role for the P gene in this process, suggesting it does not encode a key tyrosine transporter.
Area of Science:
- Cell Biology
- Biochemistry
- Genetics
Background:
- Tyrosine is essential for melanin production within melanosomes.
- The P gene's role in mouse melanogenesis and the pink-eyed dilution phenotype suggests it may code for a tyrosine transporter.
- The mechanism of tyrosine transport into melanosomes remains uninvestigated.
Purpose of the Study:
- To investigate tyrosine transport into melanosomes.
- To determine if the P gene is responsible for encoding a critical tyrosine transporter.
Main Methods:
- Utilized radiolabeled [3H]tyrosine to measure uptake in melanosome-rich fractions from normal (melan-a) and pink-eyed dilution mice.
- Assessed plasma membrane tyrosine transport kinetics (Km, Vmax) in pink-eyed dilution.
- Analyzed melanosome characteristics (density, hexosaminidase activity, pigment content).
Main Results:
- Normal and pink-eyed dilution melanosome-rich fractions exhibited similar [3H]tyrosine uptake rates.
- Plasma membrane tyrosine transport in pink-eyed dilution was found to be normal.
- Pink-eyed dilution melanosomes were characterized as immature, showing low density, high hexosaminidase activity, and lack of pigment.
Conclusions:
- A tyrosine transport system is present in the melanosomal membrane.
- The P gene does not appear to encode a tyrosine transporter of critical importance for melanosomal tyrosine uptake.
- The pink-eyed dilution phenotype is likely due to melanosome immaturity rather than a defect in tyrosine transport.