A white collar 1-like protein mediates opposite regulatory functions in Mucor circinelloides

Protein ubiquitylation plays a major role in the regulation of many cellular processes by altering the stability, localization or function of target proteins. CrgA is a protein of Mucor circinelloides that shows the characteristics of ubiquitin ligases and is involved in the regulation of carotenogenesis and asexual sporulation in this fungus. CrgA, which belongs to a poorly characterized group of proteins present in almost all eukaryotes, represses carotenogenesis through the proteolysis-independent mono- and di-ubiquitylation of Mcwc-1b, a White Collar-1-like protein which, when it is non-ubiquitylated, activates carotenogenesis. Using a proteomic approach, this work shows that the regulation of M. circinelloides vegetative development by CrgA is also mediated by Mcwc-1b, although, in this case, the non-ubiquitylated Mcwc-1b form acts as a repressor. High levels of a protein that contains a classical Rossmann-fold NAD(P)H/NAD(P)(+) binding domain for NAD(P)H binding and is similar to NmrA NADP(H) sensor-like proteins occur when Mcwc-1b is inactivated by ubiquitylation. A role for this protein in the regulation of sporulation is suggested because its over-expression suppresses the sporulation defect in a crgAΔ mutant. NmrA-like proteins are repressors that interact with GATA transcription factors and have been shown to be related to cell differentiation in Magnaporthe oryzae and Dictyostelium discoideum. This proteomic approach also revealed that CrgA regulates the carbon and energy metabolism and that Mcwc-1b is the main, but not the only, target of CrgA.