Isolation and characterization of plasma membranes from the fungus Podospora anserina

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

Mushroom-forming basidiomycetes colonize large areas in nature. Their hyphae are compartmentalized by perforated septa, which are usually covered by a septal pore cap (SPC). Here, we describe, for the first time, the composition and function of SPCs using the model system Schizophyllum commune. The SPC of S. commune was shown to consist of a proteinaceous matrix covered by a lipid membrane. The matrix was demonstrated to define the ultrastructure of the SPC and to consist of two main proteins, Spc14 and Spc33. Gene spc14 encodes a protein of 86 amino acids, which lacks known domain, signal or localization sequences. Gene spc33 encodes a 239 and a 340 amino acid variant. Both forms contain a predicted signal anchor that targets them to the ER. Immuno-localization showed the presence of Spc33 in the SPC but not in ER. From this and previous reports it is concluded that the SPC is derived from this organelle. Inactivation of spc33 resulted in loss of SPCs and the inability to close septa. The latter may well explain why vegetative growth and mushroom formation were severely reduced in strains in which spc33 was inactivated.

Subcellular fractionation of actively growing protoplasts of Saccharomyces cerevisiae

Cell homogenates obtained from partially regenerated Saccharomyces cerevisiae protoplasts were fractionated by a procedure using a combination of continuous and discontinuous sucrose gradients, under experimental conditions that minimize possible artifacts due to centrifugation and resuspension. At least five different membranous organelle fractions (plasma membrane, mitochondria, rough endoplasmic reticulum, smooth endoplasmic reticulum-like structures and small-sized particulated structures) were isolated. Subcellular fractions were characterized by assaying established marker enzymes. Radioactive labelled [(U-3H]uracil) ribosomes were also used as a further characterization criterion of the rough endoplasmic reticulum. Comparative SDS-polyacrylamide gel electrophoresis of the protein constituents of the isolated membrane-bound organelles suggest that the polypeptide pattern could also be used as an additional marker for some of these structures. Finally, subcellular distribution of chitin synthase was determined using this fractionation procedure, and two partially zymogenic enzyme pools (one inside the cell associated to particles which sediments at high speed, and the second one associated to the plasma membrane) were found.