position of the Spitzenkörper defines the center of maximal growth, the tip-most
plasma membrane area where incorporation of vesicles is maximal (Bartnik and
Sievers 1988; Hejnowicz et al. 1977; Sievers et al. 1979; Braun 1996a).
The structural integrity of the Spitzenkörper and its function as the apical
tip-growth machinery was found to be strictly dependent on actin, myosin and
numerous actin-associated proteins (Braun 1996b, 2001; Braun and Wasteneys
2000; Braun et al. 2004) While information on the arrangement and function of
the actin cytoskeleton in the apex of most other tip-growing cell types is scarce, the
role of actin in characean rhizoids and protonemata has been well characterized. A
complex array of distinct actin microfilaments was demonstrated in the apical dome
(Braun and Wasteneys 1998a, b). Fine, mostly axially arranged actin bundles focus
in an actin-dense area in the center of the Spitzenkörper in colocalization with the
dense spherical aggregate of endoplasmic-reticulum cisternae (Fig. 4.4). Myosinlike motor proteins mediate the transport of secretory vesicles along actin microfilaments that radiate out from the Spitzenkörper towards the apical membrane (Braun
1996b; Braun and Wasteneys 1998b). Such a distinct apical actin array has not been
found in any other tip-growing cell types so far.
High-pressure frozen and freeze-substituted rhizoids investigated by dual axis
electron tomography (Limbach and Braun 2008) revealed two different types of
secretory vesicles as well as microvesicles evenly distributed in the apical region of
rhizoids, whereas clathrin-coated vesicles were exclusively found in close vicinity of
the apical plasma membrane (Fig. 4.5). The latter vesicles are most likely involved in
endocytotic processes mediating the recycling of membrane and the turn-over of
membrane-bound proteins such as ion channels and receptor proteins. When the
balance of apical exocytotic and endocytotic processes was disturbed by inhibitorinduced disruption of the actin cytoskeleton, the tip-focussed distribution pattern of
calcium channels and the steep, tip-high gradient of cytoplasmic free calcium dissipated, and tip growth stopped (Braun and Richter 1999). This steep calcium gradient
dictates the incorporation pattern of secretory vesicles and regulates the spatiotemporal activity of actin-binding proteins.
The multiple functions and the dynamic nature of the actin cytoskeleton in
rhizoids and protonemata are orchestrated by numerous actin-binding proteins.
Spectrin-like proteins, ADF (actin-depolymerizing factor) and profilin were detected
in the center of the Spitzenkörper (Fig. 4.4; Braun et al. 2004). Spectrin-like proteins
participate in the structural integrity of the ER aggregate by forming crosslinks
between ER membranes and actin microfilaments (Braun 2001). Spectrins are
known to provide a mechanism for recruiting specific subsets of membrane proteins
and to form functional microdomains and, thus, they might help to create the unique
physiological conditions for the molecular mechanisms involved in gravity-sensing
and polarized growth (Braun 2001).
Furthermore, the presence of ADF and profilin in the center of the Spitzenkörper
indicates an actin polymerizing function of this central area (Braun et al. 2004).
Additional evidence comes from drug-induced disruption of the actin cytoskeleton
4.2 The Cytoskeletal Basis of Gravitropic Tip Growth
53
plasma membrane area where incorporation of vesicles is maximal (Bartnik and
Sievers 1988; Hejnowicz et al. 1977; Sievers et al. 1979; Braun 1996a).
The structural integrity of the Spitzenkörper and its function as the apical
tip-growth machinery was found to be strictly dependent on actin, myosin and
numerous actin-associated proteins (Braun 1996b, 2001; Braun and Wasteneys
2000; Braun et al. 2004) While information on the arrangement and function of
the actin cytoskeleton in the apex of most other tip-growing cell types is scarce, the
role of actin in characean rhizoids and protonemata has been well characterized. A
complex array of distinct actin microfilaments was demonstrated in the apical dome
(Braun and Wasteneys 1998a, b). Fine, mostly axially arranged actin bundles focus
in an actin-dense area in the center of the Spitzenkörper in colocalization with the
dense spherical aggregate of endoplasmic-reticulum cisternae (Fig. 4.4). Myosinlike motor proteins mediate the transport of secretory vesicles along actin microfilaments that radiate out from the Spitzenkörper towards the apical membrane (Braun
1996b; Braun and Wasteneys 1998b). Such a distinct apical actin array has not been
found in any other tip-growing cell types so far.
High-pressure frozen and freeze-substituted rhizoids investigated by dual axis
electron tomography (Limbach and Braun 2008) revealed two different types of
secretory vesicles as well as microvesicles evenly distributed in the apical region of
rhizoids, whereas clathrin-coated vesicles were exclusively found in close vicinity of
the apical plasma membrane (Fig. 4.5). The latter vesicles are most likely involved in
endocytotic processes mediating the recycling of membrane and the turn-over of
membrane-bound proteins such as ion channels and receptor proteins. When the
balance of apical exocytotic and endocytotic processes was disturbed by inhibitorinduced disruption of the actin cytoskeleton, the tip-focussed distribution pattern of
calcium channels and the steep, tip-high gradient of cytoplasmic free calcium dissipated, and tip growth stopped (Braun and Richter 1999). This steep calcium gradient
dictates the incorporation pattern of secretory vesicles and regulates the spatiotemporal activity of actin-binding proteins.
The multiple functions and the dynamic nature of the actin cytoskeleton in
rhizoids and protonemata are orchestrated by numerous actin-binding proteins.
Spectrin-like proteins, ADF (actin-depolymerizing factor) and profilin were detected
in the center of the Spitzenkörper (Fig. 4.4; Braun et al. 2004). Spectrin-like proteins
participate in the structural integrity of the ER aggregate by forming crosslinks
between ER membranes and actin microfilaments (Braun 2001). Spectrins are
known to provide a mechanism for recruiting specific subsets of membrane proteins
and to form functional microdomains and, thus, they might help to create the unique
physiological conditions for the molecular mechanisms involved in gravity-sensing
and polarized growth (Braun 2001).
Furthermore, the presence of ADF and profilin in the center of the Spitzenkörper
indicates an actin polymerizing function of this central area (Braun et al. 2004).
Additional evidence comes from drug-induced disruption of the actin cytoskeleton
4.2 The Cytoskeletal Basis of Gravitropic Tip Growth
53