Evidence that MRas1 and MRas3 proteins are associated with distinct cellular functions during growth and morphogenesis in the fungus Mucor racemosus

The filamentous fungus Mucor racemosus provides a simple and unique model s ystem for defining the function of individual ras genes in a gene family wh ich is closely related to mammalian vas genes. The current study was design ed to investigate the role of Mras1 and Mras3 in different stages of fungal morphogenesis, including sporangiospore germination, sporulation, and dimo rphic transitions, The overall patterns of Mras1 and Mras3 transcript and p rotein accumulation were markedly different but, in general, transcripts an d proteins were present at low levels during spherical growth and their acc umulated level increased severalfold during polar growth (germ tube emergen ce and elongation). In contrast to Mras1, relatively high levels of Mras3 t ranscript accumulated during sporulation and MRas3 protein accumulated in s porangiospores. Transformation of M. racemosus with an activated allele of Mras3 reduced growth rate during aerobic sporangiospore germination, while a dominant-negative allele of Mras3 caused a 40% decrease in viable asexual spores. An activated allele of Mras1 increased growth rate during sporangi ospore germination but neither activated nor dominant-negative alleles of M ras1 affected total number of asexual spores. Expression of MRas3 and MRas1 proteins appear to be subject to different regulatory mechanisms: exogenou s dibutyryl-cAMP and fusidienol caused a strong repression of the level of MRas3 protein (but not MRas1) concurrent with the inhibition of polar growt h, Differential posttranslational modification and intracellular localizati on of MRas1 and MRas3 proteins were also observed. The data strongly sugges t that Mras3 and Mras1 play different roles in regulation of cell growth an d morphogenesis in Mucor. (C) 1999 Academic Press.