Myosin Motor-Like Domain of the Class VI Chitin Synthase CsmB Is Essential to Its Functions in Aspergillus nidulans

Chitin is one of the major cell wall components of ascomycete filamentous fungi, and chitin synthesis plays important roles in the morphogenesis of hyphae. In the Aspergillus nidulans genome, there are two genes, csmA and csmB, that encode a myosin motor-like domain (MMD) at their N-termini and a chitin synthase domain (CSD) at their C-termini. In our previous studies, we found that the MMD of CsmA was required for its functionality, and that CsmA and CsmB had certain overlapping functions essential for polarized filamentous growth. In this study, we constructed a strain that produced CsmB lacking the MMD (CS Delta MB). This strain exhibited defects similar to those of the csmB deletion mutant. FLAG-tagged CS Delta MB (CS Delta MB-FLAG) did not properly localize to the hyphae. CsmB was co-immunoprecipitated with actin in vivo, whereas CS Delta MB was not. These results suggest that the MMD of CsmB is crucial for its proper localization via interaction with actin.