Abstract
ABSTRACTPollen tubes are the vehicle for sperm cell delivery to the embryo sac during fertilisation of Angiosperms. They provide an intriguing model for unravelling mechanisms of growing to extremes. The asymmetric distribution of lipids and proteins in the pollen tube plasma membrane modulates ion fluxes and actin dynamics and is maintained by a delicate equilibrium between exocytosis and endocytosis. The structural constraints regulating polarised secretion and asymmetric protein distribution on the plasma membrane are mostly unknown. To address this problem, we investigated whether ordered membrane microdomains, namely membrane rafts, might contribute to sperm cell delivery. Detergent insoluble membranes, rich in sterols and sphingolipids, were isolated from tobacco pollen tubes. MALDI TOF/MS analysis revealed that actin, prohibitins and proteins involved in methylation reactions and in phosphoinositide pattern regulation are specifically present in pollen tube detergent insoluble membranes. Tubulins, voltage-dependent anion channels and proteins involved in membrane trafficking and signalling were also present. This paper reports the first evidence of membrane rafts in Angiosperm pollen tubes, opening new perspectives on the coordination of signal transduction, cytoskeleton dynamics and polarised secretion.
Highlights
The mosaic fluid model of Singer and Nicolson (Singer and Nicolson, 1972) was recently reviewed in light of a new concept, representing cell membranes as a mosaic of highly structured microdomains, called membrane rafts, alternating with less organised regions (Lingwood and Simons, 2010)
Lipids themselves are a prerequisite for phase partition, specific protein recruitment helps these domains to compartmentalise cell processes in the plasma membrane (PM) (Schroeder et al, 1994; Brown and London, 1997) and to regulate protein sorting to different cell destinations (Muniz and Zurzolo, 2014)
Lipid raft domains are involved in endocytosis (Kirkham and Parton, 2005; Eyster et al, 2009); studies of GPI-anchored proteins showed internalisation pathways based on caveolin-coated domains (Anderson, 1998) or on the integrity of detergent insoluble microdomains (DIMs) (Sabharanjak et al, 2002)
Summary
The mosaic fluid model of Singer and Nicolson (Singer and Nicolson, 1972) was recently reviewed in light of a new concept, representing cell membranes as a mosaic of highly structured microdomains, called membrane rafts, alternating with less organised regions (Lingwood and Simons, 2010). The prerequisite for phase separation and detergent insolubility resides in the close lateral associations established between sterols, sphingolipids and highly saturated phospholipids (Ahmed et al, 1997; Brown and London, 2000; Dietrich et al, 2001; London, 2002; Silvius, 2003). The association between rigid sterol molecules and sphingolipids leads to a more organised, liquidordered phase (Lo) (Ohvo-Rekilaet al., 2002; Silvius, 2003) that coexists in the same membrane with liquid-disordered (Ld) domains (Brown and London, 1997; Edidin, 2003). Membrane microdomains take part in cell processes, such as signal transduction (Simons and Toomre, 2000) and cytoskeleton organisation (Falk et al, 2004; Chichili and Rodgers, 2009) which further promote polarised morphogenesis in animals and fungi (Cheng et al, 2001; Bagnat and Simons, 2002; Martin and Konopka, 2004)
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