Myosin Motor-Like Domain of Class VI Chitin Synthase CsmB of Aspergillus nidulans Is Not Functionally Equivalent to That of Class V Chitin Synthase CsmA

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Chitin is a major cell wall component of many filamentous fungi. Among the eight chitin synthase genes of Aspergillus nidulans, csmA and csmB encode a myosin motor-like domain (MMD) and a chitin synthase domain (CSD) at their N- and C-termini respectively. In our previous reports, we suggested that CsmA and CsmB play compensatory roles essential for polarized hyphal growth although their functions do not completely overlap, and that their MMDs are essential for their functions. In the present study, we constructed chimeric csm genes by swapping N-terminal MMD-encoding halves of csmA and csmB and studied them to identify functional differences in the MMDs. Expression of the chimeric gene encoding the MMD-including half of CsmA (MA) and the CSD-including half of CsmB thoroughly suppressed the phenotypic defects of the ΔcsmB mutant, whereas the chimeric gene encoding the MMD-including half of CsmB (MB) and the CSD-including half of CsmA did not fully suppress the defects of the ΔcsmA mutant, suggesting that MA suffices for the function of MB while MB is not functionally equivalent to MA.