Kriechbaum, C. | Search bioRxiv

bioRxiv2024

Plasma membrane and cytoplasmic compartmentalization: a dynamic structural framework required for pollen tube tip growth

Rapid, unidirectional pollen tube tip growth is essential for fertilization and is widely employed as a model of polar cell expansion, a process crucial for plant morphogenesis. Different proteins and lipids with key functions in the control of polar cell expansion are associated with distinct domains of the plasma membrane (PM) at the pollen tube tip. These domains need to be dynamically maintained during tip growth, which depends on massive secretory and endocytic membrane traffic. Very little is currently known about the regulatory and cellular mechanisms responsible for the compartmentalization of the pollen tube PM. To provide a reliable structural framework for the further characterization of these mechanisms, an integrated quantitative map was compiled of the relative positions in normally growing tobacco pollen tubes of PM domains 1) enriched in key signaling proteins or lipids, 2) displaying high membrane order, or 3) in contact with cytoplasmic structures playing important roles in apical membrane traffic. Previously identified secretory and endocytic PM domains were also included into this map. Internalization of regulatory proteins or lipids associated with PM regions overlapping with the endocytic domain was assessed based on brefeldin A (BFA) treatment. These analyses revealed remarkable aspects of the structural organization of tobacco pollen tube tips, which enhance our understanding of tip growth by providing important insights into 1) RAC/ROP signaling, 2) phosphatidylinositol 4,5-bisphosphate (PI4,5P2) metabolism and functions, 3) trafficking of signaling lipids, 4) functions of domains displaying high membrane order, and 5) Ca2+ regulation of secretion. SummaryQuantitative mapping of plasma membrane and cytoplasmic domains at the tip of elongating tobacco pollen provides important insights into regulatory and cellular mechanisms essential for tip growth.

cell biology↗

bioRxiv2021

Heterodimers of functionally divergent ARF-GEF paralogues prevented by self-interacting dimerisation domain

Functionally divergent paralogs of homomeric proteins do not form potentially deleterious heteromers, which requires distinction between self and non-self (Hochberg et al., 2018; Marchant et al, 2019; Marsh and Teichmann, 2015). In Arabidopsis, two ARF guanine-nucleotide exchange factors (ARF-GEFs) related to mammalian GBF1, named GNOM and GNL1, can mediate coatomer complex (COPI)-coated vesicle formation in retrograde Golgi-endoplasmic reticulum (ER) traffic (Geldner et al., 2003; Richter et al., 2007; Teh and Moore, 2007). Unlike GNL1, however, GNOM is also required for polar recycling of endocytosed auxin efflux regulator PIN1 from endosomes to the plasma membrane. Here we show that these paralogues form homodimers constitutively but no heterodimers. We also address why and how GNOM and GNL1 might be kept separate. These paralogues share a common domain organisation and each N-terminal dimerisation (DCB) domain can interact with the complementary fragment ({Delta}DCB) of its own and the other protein. However, unlike self-interacting DCBGNOM (Grebe et al., 2000; Anders et al., 2008), DCBGNL1 did not interact with itself nor DCBGNOM. DCBGNOM removal or replacement with DCBGNL1, but not disruption of cysteine bridges that stabilise DCB-DCB interaction, resulted in GNOM-GNL1 heterodimers which impaired developmental processes such as lateral root formation. We propose precocious self-interaction of the DCBGNOM domain as a mechanism to preclude formation of fitness-reducing GNOM-GNL1 heterodimers.

plant biology↗