Gul-1 mediates cell wall remodeling via the cot-1 pathway in Neurospora crassa

In Neurospora crassa, impaired function of the NDR kinase COT-1 results in markedly thickened cell walls. This effect is partially suppressed by inactivation of gul-1 (an RNA-binding protein involved in translational regulation of cell wall remodeling proteins). Using electron microscopy, we determined that inactivation of gul-1 also results in improved characteristics of the cot-1 cell wall and septa. These observations coincide with our findings that a 40% increase in chitin content in the cell wall of cot-1 (when compared to the wild type) was almost completely abolished in the gul-1;cot-1 double mutant. The gul-1 mutant was also found to be almost 2-fold more sensitive to a chitin synthase inhibitor (Nikkomycin Z), when compared to the wild type. We suggest that GUL1 contributes to changes in cell wall morphology and composition in a manner that confers the suppressive effect on cot-1. We found that gul-1 is involved in regulation of the expression of several cell wall remodeling genes such as glucan/chitin synthases and chitinase in a manner which is at least partially independent of the classic cell wall integrity pathway. Our RNASeq analysis revealed that GUL-1 affects transcript abundance of at least 25 genes involved in cell-wall remodeling via the COT-1 pathway. Moreover, GUL-1 was also found to regulate additional pathways such as transmembrane transport as well as amino acid metabolism. Based on catRAPID algorithm analysis mRNAs of some of the differentially-expressed cell wall-related genes have been predicted to physically interact with the GUL-1 protein. RNA antisense purification (RAP) analysis coupled with mass spectrometry is currently being employed to determine GUL-1-mRNA interactions, including the possibility that GUL-1 binds its own transcript.