Septin ring

Septins are a group of GTP-binding proteins expressed in all eukaryotic cells except plants. Different septins form protein complexes with each other. These complexes can further assemble into filaments, rings and gauzes. Assembled as such, septins function in cells by localizing other proteins, either by providing a scaffold to which proteins can attach, or by forming a barrier preventing the diffusion of molecules from one compartment of the cell to another, or in the cell cortex as a barrier to the diffusion of membrane-bound proteins. Septins are a group of GTP-binding proteins expressed in all eukaryotic cells except plants. Different septins form protein complexes with each other. These complexes can further assemble into filaments, rings and gauzes. Assembled as such, septins function in cells by localizing other proteins, either by providing a scaffold to which proteins can attach, or by forming a barrier preventing the diffusion of molecules from one compartment of the cell to another, or in the cell cortex as a barrier to the diffusion of membrane-bound proteins. Septins have been implicated in the localization of cellular processes at the site of cell division, and at the cell membrane at sites where specialized structures like cilia or flagella are attached to the cell body. In yeast cells, they compartmentalize parts of the cell and build scaffolding to provide structural support during cell division at the septum, from which they derive their name. Research in human cells suggests that septins build cages around pathogenic bacteria, that immobilize and prevent them from invading other cells. As filament forming proteins, septins can be considered part of the cytoskeleton. Apart from forming non-polar filaments, septins associate with cell membranes, the cell cortex, actin filaments and microtubules. Septins are P-Loop-NTPase proteins that range in weight from 30-65 kDa. Septins are highly conserved between different eukaryotic species. They are composed of a variable-length proline rich N-terminus with a basic phosphoinositide binding motif important for membrane association, a GTP-binding domain, a highly conserved Septin Unique Element domain, and a C-terminal extension including a coiled coil domain of varying length. Septins interact either via their respective GTP-binding domains, or via both their N- and C-termini. Different organisms express a different number of septins, and from those symmetric oligomeres are formed. For example, in humans Sept7-Sept6-Sept2-Sept2-Sept6-Sept7 form one complex, and in yeast Cdc11-Cdc12-Cdc3-Cdc10-Cdc10-Cdc3-Cdc12-Cdc11 form another one. These complexes then associate to form non-polar filaments, filament bundles, cages or ring structures in cells. Septins are found in fungi, animals, and some eukaryotic algae but are not found in plants. There are seven different septins in Saccharomyces cerevisiae. Five of those are involved in mitosis, while two (Spr3 and Spr28) are specific to sporulation. Mitotic septins (Cdc3, Cdc10, Cdc11, Cdc12, Shs1) form a ring structure at the bud neck during cell division. They are involved in the selection of the bud-site, the positioning of the mitotic spindle, polarized growth, and cytokinesis. The sporulating septins (Spr3, Spr28) localize together with Cdc3 and Cdc11 to the edges of prospore membranes. Septins form a specialised region in the cell cortex known as the septin cortex. The septin cortex undergoes several changes throughout the cell cycle: The first visible septin structure is a distinct ring which appears ~15 min before bud emergence. After bud emergence, the ring broadens to assume the shape of an hourglass around the mother-bud neck. During cytokinesis, the septin cortex splits into a double ring which eventually disappears. How can the septin cortex undergo such dramatic changes, although some of its functions may require it to be a stable structure? FRAP analysis has revealed that the turnover of septins at the neck undergoes multiple changes during the cell cycle. The predominant, functional conformation is characterized by a low turnover rate (frozen state), during which the septins are phosphorylated. Structural changes require a destabilization of the septin cortex (fluid state) induced by dephosphorylation prior to bud emergence, ring splitting and cell separation. The composition of the septin cortex does not only vary throughout the cell cycle but also along the mother-bud axis. This polarity of the septin network allows concentration of some proteins primarily to the mother side of the neck, some to the center and others to the bud site.