Disease relevance of Mlph

• In the present report, we evaluated the effect of overexpression of these mutants on melanosome, melanophilin, and myosin-Va localization in B16 melanoma cells [1].
• We herein present genetic and functional evidence that a third form of GS (GS3), whose expression is restricted to the characteristic hypopigmentation of GS, results from mutation in the gene that encodes melanophilin (Mlph), the ortholog of the gene mutated in leaden mice [2].

Psychiatry related information on Mlph

• Here we found that melanophilin directly activates the actin-activated ATPase activity of myosin Va and thus its motor activity [3].

High impact information on Mlph

• By contrast, knockdown of Slac2-a (also called melanophilin), another Rab27A-binding protein in melanocytes, caused perinuclear aggregation of melanosomes alone without altering cell shape [4].
• Rab27a binds to the melanosome first and then recruits melanophilin, which in turn recruits myosin-Va [5].
• In this study, we show that Mlp is also a microtubule plus end-tracking protein or +TIP [6].
• Collectively, our data show that Mlph is a critical component of the melanosome transport machinery and suggest that Mlph might function as part of a transport complex with Rab27a and MyoVa [7].
• However, Mlph does not contain the two Ca(2+)-binding C(2) domains found in these and other proteins involved in vesicle transport, suggesting that it represents a previously unrecognized class of Rab effectors [7].

Biological context of Mlph

• However, in contrast to Gln78Leu, Trp73Gly mutant construct neither interacts with the Rab27a effector melanophilin nor modifies melanosome distribution and cytotoxic granule exocytosis [8].
• Missense mutations in the globular tail of myosin-Va in dilute mice partially impair binding of Slac2-a/melanophilin [9].
• The murine dilute suppressor gene, dsu, was previously shown to suppress the dilute coat color phenotypes of mice homozygous for the dilute (d), leaden (ln), and ashen (ash) mutations [10].
• Interestingly, d and ash are closely linked on chromosome 9 while dsu and ln are located on chromosome 1 [11].
• Three, with differing phenotypes, mapped on Chromosome 1 between the loci of fuzzy (fz) and leaden (ln) and close to the locus of the gamma-crystallin gene cluster [12].

Anatomical context of Mlph

• Mutations in Mlph, encoding a member of the Rab effector family, cause the melanosome transport defects observed in leaden mice [7].
• We have utilized primary melanocytes to examine the interdependencies between rab27a, myosin-Va, and melanophilin [13].
• Our results showed that Rab27a-L130P cannot bind GTP, does not interact with melanophilin, and consequently cannot allow melanosome transport on the actin filaments [1].
• However, in contrast to melanocytes, we found that neither the leaden gene product (melanophilin) nor myosin Va was detectable in cytotoxic T lymphocytes [14].
• Thus, we propose that the stimulation of the interaction between melanophilin/Slac2-a and actin would allow the rapid accumulation of melanosomes in the actin-rich region of the dendrite extremities [15].

Associations of Mlph with chemical compounds

• Cyclic AMP promotes a peripheral distribution of melanosomes and stimulates melanophilin/Slac2-a and actin association [15].

Physical interactions of Mlph

• Although Slac2-a directly interacts with Rab27A and myosin Va via its N-terminal region (amino acids 1 to 146) and the middle region (amino acids 241 to 405), respectively, the functional importance of the putative actin-binding domain of the Slac2-a C terminus (amino acids 401 to 590) in melanosome transport has never been elucidated [16].

Regulatory relationships of Mlph

• The actin-activated ATPase activity of the melanocyte-type myosin Va having exon-F was significantly activated by melanophilin by 4-fold [3].

Other interactions of Mlph

• The murine dilute suppressor gene dsu suppresses the coat-color phenotype of three pigment mutations that alter melanocyte morphology, d, ash and ln [11].
• Site of beige (bg) and leaden (ln) pigment gene expression determined by recombinant embryonic skin grafts and aggregation mouse chimaeras employing sash (Wsh) homozygotes [17].
• We found that the C-terminal domain of Slac2-a is highly sensitive to low concentrations of proteases, such as trypsin and calpain, in vitro, whereas the N-terminal Rab27A-binding domain is highly resistant to these proteases [18].
• In this study, we discovered that Slac2-a and a closely related isoform, Slac2-c/MyRIP, contain multiple PEST-like sequences (potential signals for rapid protein degradation) in the myosin Va- and actin-binding domains at the C terminus [18].
• Using the Mlph R35W mutant that blocks Mlph-Rab27a interaction and Rab27a siRNA we show this interaction is required for melanosome targeting and stability of Mlph [19].

Analytical, diagnostic and therapeutic context of Mlph

• Deletion analysis and site-directed mutagenesis showed that the N-terminal Slp (synaptotagmin-like protein) homology domain of Slac2-a specifically binds Rab27A/B isoforms and that the C-terminal half directly binds the globular tail of myosin Va [20].

References