Cell polarity regulation is a central process during cell morphogenesis. The establishment and maintenance of cell polarity occurs in response to diverse external and internal signals, which control processes such as cell symmetry breaking, polarized tip growth, and cellular compartmentalization, as well as intracellular transport of RNA, proteins and organelles.
Many key aspects of the molecular basis of cell polarity regulation have been elucidated using yeast models, including Saccharomyces cerevisiae
, Schizosaccharomyces pombe
, and Candida albicans
(reviewed for example in 
), but certain facets of filamentous fungal morphogenesis are more complex and cannot be explained by the yeast paradigm 
. These most notably include: (1) the ability to simultaneously establish several axes of polarized growth from the individual spore thereby giving rise to functionally distinct cell protrusions (e.g. germ tubes and conidial anastomosis tubes [CATs]) 
, (2) the ability to permanently maintain polarized tip growth and form tubular hyphae which can achieve much higher tip growth rates than can yeasts 
, and (3) to establish interconnected germling and hyphal networks by cell fusion 
. Some of the molecular components conserved between yeasts and filamentous fungi appear to be used in different morphogenetic contexts during filamentous fungal development, and proteins no longer encoded in the yeast genome are additional key features responsible for the more complex, multicellular morphology of filamentous fungi.
The tip growth apparatus of vegetative hyphae consists of three major components: the Spitzenkörper (Spk), the polarisome and the exocyst 
. Together, they contain more than 40 different proteins 
which, in interaction with the three cytoskeletal polymers F-actin, microtubules and septins, regulate hyphal morphogenesis and tip growth 
. Targeted secretion of plasma membrane and cell wall components through the exocyst drives tip extension, and is coupled to compensatory endocytosis within a subapical collar 
, rich in F-actin patches 
. The newly emerging ‘Apical Recycling Model’ accounts for the need to balance exocytosis and endocytosis at the hyphal tip in order to control growth and cell shape, maintain high tip extension rates and recover key plasma membrane proteins (e.g. receptors) back to the growing apex 
. Therefore, recycling endocytosis can be considered a fourth key component of the hyphal tip growth apparatus.
The polarisome is involved in the establishment, maintenance and termination of polarized cell growth. Proteins known to constitute this complex in budding yeast include the three core components Spa2, Pea2 and Aip3/Bud6, as well as the formin Bni1 
. Localization and activation of Bni1 at the cell cortex requires the presence of all three core proteins 
, which together localize in an apical cap driving the directed extension of the bud, mating projection or pseudohypha. All are equally required to delocalize apical actin and terminate mating projection growth in budding yeast 
Spa2 is considered to be the central polarisome scaffolding protein that physically interacts with all other components through specific binding domains 
. Pea2 contains a predicted coiled-coil domain suggesting a possible function in targeted vesicle delivery; its precise molecular role, however, remains obscure. Nevertheless, it has been shown to display interdependent localization with Spa2, and to be required for bipolarization and mating cell fusion 
. The actin-interacting protein Aip3/Bud6 was initially identified as a protein that besides its association with actin also contains domains which suggested binding to Spa2, Pea2 and Bni1 
. The formin Bni1 is stimulated by Bud6 in a positive feedback loop and together they reinforce the assembly of robust actin cables from the cell cortex during budding and mating projection formation, and contractile actomyosin ring formation during cytokinesis 
. More recent data suggested an additional function of Bud6 in microtubule plus-end capture at the cell cortex, with contributions of formins 
. Localized assembly of these polarity regulators in the polarisome is maintained through a positive feedback loop from the Cdc42/Cdc24/Bem1 module whose components shuttle between the cytoplasm and plasma membrane 
. Due to its vital importance in cell polarity regulation, this Cdc42 GTPase module can be considered a fifth core component of the tip growth apparatus.
Homologs of Spa2, Aip3/Bud6 and Bni1 have been identified in a number of other filamentous growing fungi, and the majority already successfully localized in at least one of those species (). A homolog of Pea2 has so far only been identified in the filamentous yeast, Ashbya gossypii 
. Recent work in Neurospora crassa
has demonstrated how deletion or loss-of-function of CDC42, RAC-1 or CDC24 lead to severe defects in apical polarity and consequently hyphal morphologies 
, thereby demonstrating the vital role of this GTPase module in filamentous fungal cell polarity regulation. Although many new insights into the inner workings of the polarisome have been gained over the past decades, it is very likely that additional components localizing to this structure will be identified in the near future, revealing further details of its functional differences between yeasts and filamentous fungi.
Polarisome components in yeast and filamentous fungi.
In this study we set out to analyze the subcellular organization and dynamics of SPA-2, BUD-6 and BNI-1 in a wide range of developmental stages of Neurospora crassa, in order to characterize the filamentous fungal polarisome more comprehensively, and identify potential differences to other fungal species. Our analysis showed that during early, unicellular developmental stages the filamentous fungal polarisome closely resembles the apical cap configuration known from yeasts, but during later, multicellular developmental stages the three polarisome components SPA-2, BUD-6 and BNI-1 become spatiotemporally separated within the apical dome, and thus adopt a so far unknown polarisome architecture. Furthermore, novel polarisome-independent functions of BUD-6 and BNI-1 have been identified, including the maintenance of Spitzenkörper integrity, cell fusion, septum formation and cytokinesis.