CsmA, a Class V Chitin Synthase with a Myosin Motor-like Domain, Is Localized through Direct Interaction with the Actin Cytoskeleton inAspergillus nidulans

Abstract

One of the essential features of fungal morphogenesis is the polarized synthesis of cell wall components such as chitin. The actin cytoskeleton provides the structural basis for cell polarity in Aspergillus nidulans, as well as in most other eukaryotes. A class V chitin synthase, CsmA, which contains a myosin motor-like domain (MMD), is conserved among most filamentous fungi. The ⌬csmA null mutant showed remarkable abnormalities with respect to cell wall integrity and the establishment of polarity. In this study, we demonstrated that CsmA tagged with 9؋ HA epitopes localized near actin structures at the hyphal tips and septation sites and that its MMD was able to bind to actin. Characterization of mutants bearing a point mutation or deletion in the MMD suggests that the interaction between the MMD and actin is not only necessary for the proper localization of CsmA, but also for CsmA function. Thus, the finding of a direct interaction between the chitin synthase and the actin cytoskeleton provides new insight into the mechanisms of polarized cell wall synthesis and fungal morphogenesis. INTRODUCTIONThe filamentous fungus Aspergillus nidulans grows by generating ordered networks of filaments, or hyphae, which form a mycelium. Chitin, a ␤-1,4-linked homopolymer of N-acetylglucosamine (GlcNAc), is one of the major structural components of the fungal cell wall. The temporal and spatial regulation of its metabolism is very important for tip growth and the morphogenesis of a number of filamentous fungi (Bulawa, 1993;Cid et al., 1995). Actin is concentrated at the growing apices and sites of septum formation, where cell wall synthesis or septal synthesis is active (Harris et al., 1994;Momany and Hamer, 1997). Cytochalasin A, an inhibitor of actin polymerization, has been shown to induce the swelling of the hyphal tips and to block septum formation (Harris et al., 1994;Torralba et al., 1998). Thus, the actin cytoskeleton plays important roles in the determination of hyphal polarity (Torralba and Heath, 2001). Chitin synthases, membranebound proteins that catalyze the polymerization of GlcNAc from UDP-GlcNAc as a substrate, have been classified into at least six groups, classes I to VI, on the basis of the structures of their conserved region (Roncero, 2002). We have isolated five chitin synthase genes from A. nidulans and designated them as chsA, chsB, chsC, chsD, and csmA, and their gene products belong to classes II, III, I, IV, and V, respectively (Motoyama et al., 1994(Motoyama et al., , 1996Yanai et al., 1994;Fujiwara et al., 1997). The csmA (chitin synthase with a myosin motor-like domain) gene encodes a protein (1852 amino acids) consisting of an N-terminal myosin motor-like domain (MMD, ϳ800 amino acids) and a C-terminal chitin synthase domain (CSD, ϳ750 amino acids). Myosins are known as mechanoenzymes that convert chemical energy, liberated through ATP hydrolysis, into a mechanical force that is directed along actin filaments. The MMD of CsmA bears some consensus motifs of myosins, such as P-loop, Switch I, and...

Identification of Chitin in the Prismatic Layer of the Shell and a Chitin Synthase Gene from the Japanese Pearl Oyster,Pinctada fucata

“…30,31) This myosin head domain is thought to interact with actin to work with cytoskeltal function. 32) To investigate the evolutionary relationship between PfCHS1 and known chitin synthases, a phylogenetic tree was constructed with the Clustal W program 33) with the Neighbor Joining (NJ) algorithm 34) (Fig. 7).…”

Studies on the Chemical Structures of Organic Matrices and Their Functions in the Biomineralization Processes of Molluscan Shells

“…In addition to these three domains, a homology search indicated that the N-terminal domain (1-758 a.a.) of PfCHS1 has high similarity with the myosin head domain of ScunM, unconventional myosin obtained from scallop mantle tissue, and ArCHS1 identified from Atrina rigida, a chitin synthase gene. This myosin head domain is thought to interact with actin in cytoskeletal function (Fujiwara et al 1997;Park et al 1999;Takeshita et al 2005). which originate from the regular alternation of organic matrix sheets and platy aragonite crystals (Figs.…”

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

“…The strains were inoculated on MMGuu plates and incubated for 2 d, and the total number of balloons was determined as described previously. 16) Other methods. Chitin content measurement was performed as described previously.…”

“…16,17) In the present study, we examined the à The ratios of these data for the ÁcsmA/argB1, MACBÁA1, and MBCAÁA1 strains to those for the ÁcsmA/csmA1 strain are shown in parentheses. a Data are mean AE SD (n ¼ 3).…”

“…The MMDs of CsmA and CsmB bind to actin filaments in vitro and the interaction between MMD and actin is important for proper localization and normal functioning. 7,16,17) These findings suggest that CsmA and CsmB have overlapping functions that are essential for hyphal tip growth. The phenotypes of the csmA and csmB deletion mutants differ in some respects, and overexpression of csmA and of csmB does not suppress the defects of the csmB and csmA deletion mutants respectively.…”