Calmodulin and calmodulin-binding proteins of Rhodosporidium toruloides, a basidiomycetous yeast

Calmodulin (CaM)-interacting proteins of Rhodosporidium toruloides, a basidiomycetous yeast, were studied. CaM-binding proteins isolated from the soluble cell extract of vegetative cells and gamete cells (mating-pheromone-treated cells) by CaM-Sepharose affinity chromatography were analysed by sodium dodecylsulphate-polyacrylamide gel electrophoresis to compare the molecular species of CaM-binding proteins of the two cell types. The major CaM-binding proteins of the vegetative cells had molecular masses of 103 and 96 kDa. In the pheromone-treated cells, in addition to the bands seen in the vegetative cells, prominent bands of 40 and 38 kDa and minor bands of 92 and 60 kDa were present. The bands of CaM-binding proteins present in both cell types were more intense in the gamete cells. CaM-binding proteins of total cell extracts were also detected on acrylamide gels by a gel overlay method using ¹²⁵I-CaM as the probe. The results obtained by the two procedures suggested that various CaM-binding proteins are present in the yeast cell, and the expression of many CaM-binding protein genes is altered by the mating pheromone. Ca²⁺- and CaM-dependent protein kinase activity was detected in the CaM-affinity-purified proteins. After incubation under conditions for protein phosphorylation by the kinase, the enzyme no longer required the activators, indicating that the enzyme activity is modulated by endogenous phosphorylation. A protein of 60 kDa was labelled by endogenous phosphorylation in a Ca²⁺/CaM-dependent manner, indicating that the labelled protein may be the Ca²⁺/CaM-dependent protein kinase or else be involved in its regulation.