A mutation in Rab27a causes the vesicle transport defects observed in ashen mice

Abstract

The dilute (d), leaden (ln), and ashen (ash) mutations provide a unique model system for studying vesicle transport in mammals. All three mutations produce a lightened coat color because of defects in pigment granule transport. In addition, all three mutations are suppressed by the semidominant dilute-suppressor (dsu), providing genetic evidence that these mutations function in the same or overlapping transport pathways. Previous studies showed that d encodes a major vesicle transport motor, myosin-VA, which is mutated in Griscelli syndrome patients. Here, using positional cloning and bacterial artificial chromosome rescue, we show that ash encodes Rab27a. Rab GTPases represent the largest branch of the p21 Ras superfamily and are recognized as key players in vesicular transport and organelle dynamics in eukaryotic cells. We also show that ash mice have platelet defects resulting in increased bleeding times and a reduction in the number of platelet dense granules. These defects have not been reported for d and ln mice. Collectively, our studies identify Rab27a as a critical gene for organelle-specific protein trafficking in melanocytes and platelets and suggest that Rab27a functions in both MyoVa dependent and independent pathways.

Cortical actin recovery at the immunological synapse leads to termination of lytic granule secretion in cytotoxic T lymphocytes

“…CTLs from ashen mice, which have a mutation leading to the loss of RAB27a, are unable to secrete lytic granules and kill target cells (51)(52)(53). In these CTLs, cortical actin density remained reduced at the immunological synapse long after most WT CTLs had degranulated and recovered their cortical actin.…”

The Dilute Locus and Griscelli Syndrome: Gateways Towards a Better Understanding of Melanosome Transport

“…This is, for an as yet unknown reason, not the case in the phenotype of RAB27A-mutated Griscelli patients who do not display a bleeding disorder. In view of this platelet deficiency, Wilson et al (25) suggest that RAB27A would be a good candidate for mutation screening in patients with Hermansky-Pudlak syndrome (OMIM entry c 203300), an autosomal recessive syndrome with tyrosinase-positive oculocutaneous albinism, platelet dense granule storage pool deficiency and accumulation of ceroid pigment in lysosomes. Recent experiments reveal that the phenotype of primary epidermal melanocytes from RAB27A-mutated Griscelli patients and ashen mice could be rescued after transfection and re-expression of the active GTP bound wild type RAB27A (26,27), confirming that the Griscelli and ashen locus encode RAB27A.…”

Varp Is a Novel Rab32/38-binding Protein That Regulates Tyrp1 Trafficking in Melanocytes

“…The best characterized Rab isoform in melanocytes is Rab27A, which is involved in melanosome transfer from microtubules to actin filaments through interaction with the melanocyte-specific effector Slac2-a/melanophilin Wu et al, 2002;Kuroda et al, 2003;Hume et al, 2006;reviewed in Fukuda, 2005) and melanosome anchoring to the plasma membrane through interaction with another Rab27A effector, Slp2-a (Kuroda and Fukuda, 2004). Thus, dysfunctions of Rab27A cause pigment dilution in human type II Griscelli syndrome patients (Ménasché et al, 2000) and diluted coat color of the mouse ashen (Wilson et al, 2000) because of a defect in melanosome transport.…”