A Cyclic GMP-Dependent K+Channel in the Blastocladiomycete Fungus Blastocladiella emersonii

Abstract

ABSTRACTPhototaxis in flagellated zoospores of the aquatic fungusBlastocladiella emersoniidepends on a novel photosensor,Blastocladiella emersoniiGC1 (BeGC1), comprising a type I (microbial) rhodopsin fused to a guanylyl cyclase catalytic domain, that produces the conserved second messenger cyclic GMP (cGMP). The rapid and transient increase in cGMP levels during the exposure of zoospores to green light was shown to be necessary for phototaxis and dependent on both rhodopsin function and guanylyl cyclase activity. It is noteworthy that BeGC1 was localized to the zoospore eyespot apparatus, in agreement with its role in the phototactic response. A putative cyclic nucleotide-gated channel (BeCNG1) was also identified in the genome of the fungus and was implicated in flagellar beating via the action of a specific inhibitor (l-cis-diltiazem) that compromised zoospore motility. Here we show thatB. emersoniiexpresses a K+channel that is activated by cGMP. The use of specific channel inhibitors confirmed the activation of the channel by cGMP and its K+selectivity. These characteristics are consistent with the function of an ion channel encoded by theBeCNG1gene. Other blastocladiomycete fungi, such asAllomyces macrogynusandCatenaria anguillulae, possess genes encoding a similar K+channel and the rhodopsin–guanylyl cyclase fusion protein, while the genes encoding both these proteins are absent in nonflagellated fungi. The presence of these genes as a pair seems to be an exclusive feature of blastocladiomycete fungi. Taken together, these data demonstrate that theB. emersoniicGMP-activated K+channel is involved in the control of zoospore motility, most probably participating in the cGMP-signaling pathway for the phototactic response of the fungus.