Septal Pore Cap Protein SPC18, Isolated from the Basidiomycetous Fungus Rhizoctonia solani , Also Resides in Pore Plugs

Abstract

The hyphae of filamentous fungi are compartmentalized by septa that have a central pore. The fungal septa and septum-associated structures play an important role in maintaining cellular and intrahyphal homeostasis. The dolipore septa in the higher Basidiomycota (i.e., Agaricomycotina) are associated with septal pore caps. Although the ultrastructure of the septal pore caps has been studied extensively, neither the biochemical composition nor the function of these organelles is known. Here, we report the identification of the glycoprotein SPC18 that was found in the septal pore cap-enriched fraction of the basidiomycetous fungus Rhizoctonia solani. Based on its N-terminal sequence, the SPC18 gene was isolated. SPC18 encodes a protein of 158 amino acid residues, which contains a hydrophobic signal peptide for targeting to the endoplasmic reticulum and has an N-glycosylation motif. Immunolocalization showed that SPC18 is present in the septal pore caps. Surprisingly, we also observed SPC18 being localized in some plugs. The data reported here strongly support the hypothesis that septal pore caps are derived from endoplasmic reticulum and are involved in dolipore plugging and, thus, contribute to hyphal homeostasis in basidiomycetous fungi.

The septal pore cap is an organelle that functions in vegetative growth and mushroom formation of the wood‐rot fungus Schizophyllum commune

“…Immuno‐localization confirmed the presence of Spc33 in the SPC of S. commune but also showed its absence in ‘regular’ ER. This was also previously observed for the SPC18 protein of R. solani (van Driel et al. , 2008).…”

“…This, for the first time, shows genetic evidence for a connection between the SPC and plugging of septa. Previously, we could show by immuno‐detection that the SPC18 protein of R. solani was located both in the SPC and in the plug that closes a septum (van Driel et al. , 2008).…”

“…, 1998). This and the fact that ER markers (van Driel et al. , 2008) and zinc‐iodine osmium tetroxide (Müller et al.…”

“…Although the ultrastructure of SPCs is well studied, their composition, dynamics and function are largely unknown. Recently, we have shown that the SPC18 protein of Rhizoctonia solani not only resides in the SPC but also can be found in the plug that closes the septum upon damage (van Driel et al. , 2008).…”

“…At its base the SPC is connected to the endoplasmic reticulum (ER) (Girbardt, 1961;Moore and Patton, 1975;Müller et al, 1998). This and the fact that ER markers (van Driel et al, 2008) and zinc-iodine osmium tetroxide (Müller et al, 1998; stain the SPC region suggest that the SPC is derived from this organelle.…”