THE NEUROSPORA CRASSA COT1 KINASE - A REGULATOR OF POLAR GROWTH: INTERACTIONS WITH TYPE 2A PHOSPHATASE AND THE RNA-BINDING PROTEIN GUL1

COT1 is the founding member of the nuclear Dbf2-related (NDR) kinase family, whose members are involved in regulation of polar growth in many eukaryotic cells. Inactivation of cot-1 or altering COT1 phosphorylation states affects hyphal elongation and branching in N. crassa. MPS One Binder (MOB) proteins 2A/B are coactivators of COT1 and their interaction with COT1 is affected by the phosphorylation state of the kinase. We have determined the presence of a physical interaction between the type 2A phosphoprotein phosphatase (PP2A) catalytic and RGB1 regulatory subunits with COT1, suggesting that PP2A is a counter-acting phosphatase in the COT1 pathway. In addition, RGB1::GFP localizes to hyphal tips and along the plasma membrane, in a manner similar to COT1. Consequences of COT1 inactivation include cell wall abnormalities, concomitant with altered expression of genes encoding cell wall remodeling enzymes. At least part of these downstream effects are mediated by the COT1 substrate GUL1 [a homologue of the yeast SSD1p (Suppressor of SIT4 Deletion)]. Inactivation of gul-1 partially suppresses the cot-1 phenotype along with curbing the increased (20-250%) expression of chitin synthase (chs) 1-7, glucan synthase 1 (gls-1) and the chitinase gh18-5. Evidence concerning the gls-1 expression pattern suggests that COT1, via GUL, can at least partially bypass the classic cell wall integrity pathway. We conclude that the severity of the cot-1 phenotype is dependent on COT1 phosphorylation state and interactions with other COT1 complex components and that changes in cell wall remodeling machinery which are, at least in part, regulated by GUL1, contribute to the phenotypic alterations observed.