The mechanistic basis of self-fusion between conidial anastomosis tubes during fungal colony initiation

Abstract

The main model for studying the mechanistic basis of hyphal self-fusion is the conidial anastomosis tube (CAT) system of Neurospora crassa. CATs are specialized cell protrusions/short hyphae produced during colony initiation. They grow chemotropically towards each other and fuse to form interconnected networks of conidial germlings. CAT fusion in N. crassa is an excellent model for hyphal fusion because it is easy to analyse by live-cell imaging and is well suited for mutant analyses and experimental perturbation using pharmacological agents. ∼ 40 mutants compromised at different stages of CAT fusion have been characterized. The CAT inducer and chemoattractant are, as yet, unidentified but have been proposed to be the same self-signalling peptide. CAT fusion requires F-actin but not microtubules, and the polarisome protein complex plays an important role in cell polarity regulation during different stages of the process. Self-signalling, in which genetically identical CATs recognize each other as different, involves what has been coined the ‘ping–pong mechanism’. This entails two CATs repeatedly switching their physiological states by the oscillatory recruitment of the proteins MAK-2 and SO to CAT tips as they grow chemotropically towards each other. Once CATs make contact they adhere and the intervening cell wall is remodelled and degraded. This is followed by the merging of the two CAT plasma membranes and the formation of a fusion pore that results in cytoplasmic continuity being achieved between the fused CATs. Mutant analyses have implicated a range of other signalling pathways and processes involved in different stages of CAT fusion. These include: the Rho GTPases CDC-42 and RAC-1; the STRIPAK complex; the cell wall integrity MAP kinase pathway; redox signalling; endocytosis; and five transcription factors.