CDC-42 and RAC-1 regulate opposite chemotropisms inNeurospora crassa

Abstract

Cell polarization and fusion are crucial developmental processes that occur in response to intracellular and extracellular signals. Asexual spores (conidia) of the mold Neurospora crassa differentiate two types of polarized cell protrusions, germ tubes and conidial anastomosis tubes (CATs), which exhibit negative and positive chemotropism, respectively. We provide the first evidence that shared and separate functions of the Rho-type GTPases CDC-42 and RAC-1 regulate these opposite chemotropisms. We demonstrate that RAC-1 is essential for CAT formation and cell fusion, whereas CDC-42 is necessary and sufficient for normal germ tube development. Cdc42-Rac-interactive-binding (CRIB) reporters were constructed to exclusively label locally activated GTP-bound GTPases. Time course analyses showed that repositioning of these activated GTPase clusters within germ tube and CAT tip apices controls directional growth in the absence of a tip-localized vesicle supply center (Spitzenkö rper). We propose a model in which the local assembly of a plasma-membraneassociated GTPase-PAK-MAPK signaling platform regulates chemoattractant perception and secretion in order to synchronize oscillatory cell-cell communication and directional CAT tip growth.

Actin-Based Transport Adapts Polarity Domain Size to Local Cellular Curvature

“…Accordingly, recent theoretical models predict the existence of stability conditions on the adaptation of Cdc42-GTP domain size to cell geometry required to maintain a stable diameter over generations of dividing cells [12]. Finally, by analyzing images of active-Cdc42 domains in other cell types, we found that our scaling could be valid over a 100-fold range in radii of curvature ( Figures S4C and S4D) [2][3][4][5][6]. Because most eukaryotic cells share the use of Rho GTPases and actin-based systems to cluster functional cortical domains, our study may have uncovered a novel generic design for morphogenesis.…”

“…Similarly, cortical polarity domains built around the active form of conserved Rho-GTPases, such as Cdc42p, exhibit widths that may range over two orders of magnitudes in cells with different sizes and shapes [2][3][4][5][6]. The establishment of such domains typically involves positive feedback loops based on reaction-diffusion and/or actin-mediated vesicle transport [3,7,8].…”

Gradient Sensing: Engineering the Yeast Love Affair

“…Their conclusion is clear: the model in which the release and sensing of the pheromone are co-localized with the polarity patch outperforms other models and its simulated mating efficiency reaches that of the wild-type yeast population. It is interesting that in N. crassa the analogs of the polarity patches detected by the activity of Cdc42 and another small GTPase, Rac-1, coincide with the sites of signal reception as reported by the localization of the signal-responding MAP kinase Mak-2 (the Fus3 homologue) [18,19]. It is tempting to speculate that, even in large filamentous fungi, there exist localized zones of heightened signal sensing (and, very likely, release) that coincide with and are defined by the self-organized clusters of GTPase activity.…”

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

“…Homologues of the Rho-type GTPases Cdc42 and Rac1 have been studied in A. nidulans (ModA and RacA), N. crassa (CDC-42 and RAC-1), and U. maydis (Cdc42 and Rac1) (86)(87)(88). Both Cdc42 and Rac1 share at least one overlapping function that is required for polarity establishment.…”