A Role for Myosin 1e in Cortical Granule Exocytosis in Xenopus Oocytes

Abstract

Xenopus oocytes undergo dynamic structural changes during maturation and fertilization. Among these, cortical granule exocytosis and compensatory endocytosis provide effective models to study membrane trafficking. This study documents an important role for myosin1e in cortical granule exocytosis. Myosin1e is expressed at the earliest stage that cortical granule exocytosis can be detected in oocytes. Prior to exocytosis, myosin1e relocates to the surface of cortical granules. Overexpression of myosin1e augments the kinetics of cortical granule exocytosis, whereas tail-derived fragments of myosin1e inhibit this secretory event (but not constitutive exocytosis). Finally, intracellular injection of myosin1e antibody inhibits cortical granule exocytosis. Further experiments identified cysteine string proteins as interacting partners for myosin1e. As constituents of the membrane of cortical granules, cysteine string proteins are also essential for cortical granule exocytosis. Future investigation of the link between myosin1e and cysteine string proteins should help to clarify basic mechanisms of regulated exocytosis.A recent analysis concluded that the eukaryotic myosin superfamily includes 37 discrete types of myosins that are distinguished by the auxiliary domains linked to the ATP/actinbinding core of these motor proteins (1). Although the role of vertebrate, skeletal muscle myosins is well established, the specific function of the majority of the non-muscle, unconventional myosins remains unclear. To investigate the role of individual myosins in dynamic cellular events, we initiated studies using immature oocytes and eggs of Xenopus laevis (2, 3). These cells have several useful advantages. (i) They harbor mRNA encoding a variety of unconventional myosins (3). (ii) They undergo distinctive structural and functional changes in response to specific signaling events, including hormone-induced maturation or the induction of cortical granule exocytosis (4 -9). (iii) The large size and relative ease of manipulation of these cells facilitate biochemical and ultrastructural studies, as well as dynamic imaging experiments.In a recent study, we observed that myosin 1c (Myo1c) 2 is involved in the compression of actin coats that surround the large endosomes that are produced in the aftermath of cortical granule exocytosis in Xenopus eggs (3). Because a second type 1 myosin, myosin 1e (Myo1e; see ref. 10 for nomenclature), is present in Xenopus oocytes, we were interested whether this motor protein also has a role during cortical granule exocytosis or the ensuing process of compensatory endocytosis. Myo1e, like other myosins-1, has an NH 2 -terminal actin and ATPbinding motor domain, followed by a central calmodulin-binding IQ domain and a COOH-terminal tail. The tail is composed of a basic MyTH1 (Myosin Tail Homology 1) domain that binds anionic phospholipids, and a glycine-proline-rich MyTH2 region of unknown function (11). In addition, the tails of myosins 1e and 1f include a COOH-terminal Src homology 3 (SH3) domain that ...

Membrane traffic in the secretory pathway

“…Myosin I Myo1c GLUT4-positive vesicles Under the control of Ral A [80,81,84,114] Myo1f Granule-containing b2 integrin Unknown [79] Myo1e Cortical granules Cystein string protein [77,78] Myosin II Myo2a Trans-Golgi network vesicles Unknown [93] Cytolytic granules Unknown [94] Myo2b Golgi apparatus Unknown [91] Myosin …”

“…They have proposed that MYOA plays a structural role [76]. Another long-tail myosin 1, Myosin 1e (Myo1e), has been shown to control the regulated secretion of cortical granules in response to elevated calcium upon fertilization in Xenopus oocytes [77]. Its recruitment on cortical granules prior to exocytosis is mediated by its MyTH1 and MyTH2 domains, and its MyTH2 domain associates with assembling actin filaments in vivo [78].…”

Control of cell membrane tension by myosin-I

“…Experiments performed in multiple eukaryotic model systems have implicated class I myosins in various aspects of membranerelated events including phagocytosis (29)(30)(31)(32)(33), endocytosis (34)(35)(36)(37), exocytosis (38,39), and membrane recycling (40). Although our current data set does not allow us to rule out the possibility that perturbations in membrane trafficking may contribute to the changes in membrane tension observed in our experiments, our results do provide strong support for a model where class I myosins play a direct role in the control of membrane tension, by contributing to adhesion between the plasma membrane and underlying actin cytoskeleton.…”

Unconventional myosins acting unconventionally

“…Myosins-1 also control F-actin during both exo- and endocytosis of cortical granule membranes in Xenopus eggs [34–36] (Figure 1c). Disruption of myosin-1c function results in abnormal actin assembly during exocytosis, which limits fusion-pore expansion.…”