Crystal structure of the stomatin operon partner protein from Pyrococcus horikoshii indicates the formation of a multimeric assembly

The SPFH Protein Superfamily in Fungi: Impact on Mitochondrial Function and Implications in Virulence

Integral membrane proteins from the ancient SPFH (stomatin, prohibitin, flotillin, HflK/HflC) protein superfamily are found in nearly all living organisms. Mammalian SPFH proteins are primarily associated with mitochondrial functions but also coordinate key processes such as ion transport, signaling, and mechanosensation. In addition, SPFH proteins are required for virulence in parasites. While mitochondrial functions of SPFH proteins are conserved in fungi, recent evidence has uncovered additional roles for SPFH proteins in filamentation and stress signaling. Inhibitors that target SPFH proteins have been successfully used in cancer and inflammation treatment. Thus, SPFH proteins may serve as a potential target for novel antifungal drug development. This review article surveys SPFH function in various fungal species with a special focus on the most common human fungal pathogen, Candida albicans.

Inactive dimeric structure of the protease domain of stomatin operon partner protein

The N-terminal region of the stomatin operon partner protein (STOPP) PH1510 (1510-N) from the hyperthermophilic archaeon Pyrococcus horikoshii is a serine protease with a catalytic Ser-Lys dyad (Ser97 and Lys138) and specifically cleaves the C-terminal hydrophobic region of the p-stomatin PH1511. In a form of human hemolytic anemia known as hereditary stomatocytosis, stomatin is deficient in the erythrocyte membrane owing to mis-trafficking. Stomatin is thought to act as an oligomeric scaffolding protein to support cell membranes. The cleavage of stomatin by STOPP might be involved in a regulatory system. Several crystal structures of 1510-N have previously been determined: the wild type, the K138A mutant and its complex with a substrate peptide. Here, the crystal structure of the S97A mutant of 1510-N (1510-N S97A) was determined at 2.25 Å resolution. The structure contained two 1510-N S97A molecules in the asymmetric unit. On the superposition of one monomer of the 1510-N S97A and wild-type dimers, the S97A C^α atom of the other monomer of 1510-N S97A deviated by 23 Å from that of the wild type. This result indicates that 1510-N can greatly change the form of its dimer. Because of crystallographic symmetry in space group P6₅, a sixfold helical structure is constructed using the 1510-N dimer as a basic unit. This helical structure may be common to STOPP structures.

The lipid raft markers stomatin, prohibitin, flotillin, and HflK/C (SPFH)-domain proteins form an operon with NfeD proteins and function with apolar polyisoprenoid lipids

SPFH-domain proteins are found in almost all organisms across three domains: archaea, bacteria, and eukaryotes. In eukaryotic organelles, their subfamilies exhibit overlapping distribution and functions; thus, the rationality of annotation to discriminate these subfamilies remains unclear. In this review, the binding ability of prokaryotic SPFH-domain proteins towards nonpolar polyisoprenoides such as squalene and lycopene, rather than cholesterol, is discussed. The hydrophobic region at the C-terminus of SPFH-domain proteins constitutes the main region that binds apolar polyisoprenoid lipids as well as cholesterol and substantively contributes towards lipid raft formation as these regions are self-assembled together with specific lipids. Because the scaffolding proteins caveolins show common topological properties with SPFH-domain proteins such as stomatin and flotillin, the α-helical segments of stomatin proteins can flexibly move along with the membrane surface, with such movement potentially leading to membrane bending via lipid raft clustering through the formation of high order homo-oligomeric complexes of SPFH-domain proteins. We also discuss the functional significance and ancient origin of SPFH-domain proteins and the NfeD protein (STOPP) operon, which can be traced back to the ancient living cells that diverged and evolved to archaea and bacteria. Based on the molecular mechanism whereby the STOPP-protease degrades the C-terminal hydrophobic clusters of SPFH-domain proteins, it is conceivable that STOPP-protease might control the physicochemical properties of lipid rafts.

Proteolytic systems of archaea: slicing, dicing, and mincing in the extreme

Archaea are phylogenetically distinct from bacteria, and some of their proteolytic systems reflect this distinction. Here, the current knowledge of archaeal proteolysis is reviewed as it relates to protein metabolism, protein homeostasis, and cellular regulation including targeted proteolysis by proteasomes associated with AAA-ATPase networks and ubiquitin-like modification. Proteases and peptidases that facilitate the recycling of peptides to amino acids as well as membrane-associated and integral membrane proteases are also reviewed.