Myrip uses distinct domains in the cellular activation of myosin VA and myosin VIIA in melanosome transport

Abstract

Myrip is a Rab27a and MyosinVIIa (MyoVIIa) linking protein that may regulate melanosome transport in the retinal pigment epithelium (RPE). Myrip also binds MyosinVa (MyoVa) in vitro however it is unclear whether this interaction is of sufficient affinity to be physiologically relevant. Here, we addressed the questions of whether Myrip interacts with MyoVa in cells and the molecular basis of cellular activation of MyoVa and MyoVIIa by Myrip. To answer these questions we used melanosome transport in skin melanocytes and RPE cells as read-outs of MyoVa and MyoVIIa activity. We found that Myrip recruits and activates MyoVa on skin melanosomes with similar efficiency to the established MyoVa activator Melanophilin (Mlph). Mutagenesis showed that a Myrip-Mlph conserved amphipathic helix (MMAH) is essential for MyoVa interaction while other Myrip regions, including the MyoVa exon F binding domain equivalent, play non-essential roles in this interaction. This suggests that, in contrast to Mlph, Myrip interacts with MyoVa lacking melanocyte-specific exon F. Parallel studies of RPE melanosome transport reveal that Myrip-specific inserts, but not the MMAH, are essential for MyoVIIa activation. We conclude that Myrip is a versatile Rab27a-associated myosin-activating protein that mediates cellular activation of MyoVa and MyoVIIa via non-overlapping domains.