RNase T1 is the best known representative of a large family of ribonucleolytic proteins secreted by fungi, mostly Aspergillus and Penicillium specie

A potent hemolytic polypeptide, sticholysin II, has been purified to homogeneity from the sea anemone Stichodactyla helianthus. The protein produces leakage of aqueous contents of model lipid vesicles composed of either phosphatidylcholine or sphingomyelin if cholesterol is present in these membranes. The leakage has been analyzed by measuring the dequenching of the fluorescent dye 8‐aminonaphthalene‐1,3,6‐trisulfonic acid, coencapsulated with its quencher N,N′‐p‐xylenebispyridinium bromide, upon dilution of the vesicle contents into the external medium. The protein displays a maximum effect on vesicles containing 20−25 % cholesterol. Leakage is also produced in vesicles composed of mixtures of phosphatidylcholine and sphingomyelin, the maximum effect being observed for 20−30 % sphingomyelin molar content. The extent of the leakage is dependent on the molecular mass of the vesicle entrapped solutes in the range 445−960 Da. This suggests the involvement of a pore of about 1 nm in diameter based on the limiting size observed for the leakage of the different solutes. Oligomerization of the protein is apparently involved in the membrane permeabilization, based on the kinetic analysis of the leakage process which is shown to proceed through an all‐or‐none mechanism.

Fungi establish a complex network of biological interactions with other organisms in nature. In many cases, these involve the production of toxins for survival or colonization purposes. Among these toxins, ribotoxins stand out as promising candidates for their use in biotechnological applications. They constitute a group of highly specific extracellular ribonucleases that target a universally conserved sequence of RNA in the ribosome, the sarcin-ricin loop. The detailed molecular study of this family of toxic proteins over the past decades has highlighted their potential in applied research. Remarkable examples would be the recent studies in the field of cancer research with promising results involving ribotoxin-based immunotoxins. On the other hand, some ribotoxin-producer fungi have already been studied in the control of insect pests. The recent role of ribotoxins as insecticides could allow their employment in formulas and even as baculovirus-based biopesticides. Moreover, considering the important role of their target in the ribosome, they can be used as tools to study how ribosome biogenesis is regulated and, eventually, may contribute to a better understanding of some ribosomopathies.

Actinoporins are a family of 20-kDa, basic proteins isolated from sea anemones, whose activity is inhibited by preincubation with sphingomyelin. They are produced in monomeric soluble form but, when binding to the plasma membrane, they oligomerize to produce functional pores which result in cell lysis. Equinatoxin II (EqtII) from Actinia equina and Sticholysin II (StnII) from Stichodactyla helianthus are the actinoporins that have been studied in more detail. Both proteins display a β-sandwich fold composed of 10 β-strands flanked on each side by two short α-helices. Twodimensional crystallization on lipid monolayers has allowed the determination of low-resolution models of tetrameric structures distinct from the pore. However, the actual structure of the pore is not known yet. Wild-type EqtII and StnII, as well as a nice collection of natural and artificially made variants of both proteins, have been produced in Escherichia coli and purified. Their characterization has allowed the proposal of a model for the mechanism of pore formation. Four regions of the actinoporins structure seem to play an important role. First, a phosphocholine-binding site and a cluster of exposed aromatic residues, together with a basic region, would be involved in the initial interaction with the membrane, whereas the amphipathic N-terminal region would be essential for oligomerization and pore formation. Accordingly, the model states that pore formation would proceed in at least four steps: Monomer binding to the membrane interface, assembly of four monomers, and at least two distinct conformational changes driving to the final formation of the functional pore.