A Key Piece of the Puzzle: The central tetramer of the Saccharomyces cerevisiae septin protofilament and Its Implications for Self-Assembly

Septins, often described as the fourth component of the cytoskeleton, are structural proteins found in a vast variety of living beings. They are related to small GTPases and thus, generally, present GTPase activity which may play an important (although incompletely understood) role in their organization and function. Septins polymerase into long non-polar filaments, in which each subunit interacts with two others by alternating interfaces, NC and G. In Saccharomyces cerevisiae four septins are organized in the following manner, [Cdc11-Cdc12-Cdc3-Cdc10- Cdc10-Cdc3-Cdc12-Cdc11]n in order to form filaments. Although septins were originally discovered in yeast and much is known regarding their biochemistry and function, only limited structural information about them is currently available. Here we present crystal structures of Cdc3/Cdc10 which provide the first view of the physiological interfaces formed by yeast septins. The G-interface has properties which place it in between that formed by SEPT2/SEPT6 and SEPT7/SEPT3 in human filaments. Switch I from Cdc10 contributes significantly to the interface, whereas in Cdc3 it is largely disorded. However, the significant negative charge density of the latter suggests it may have a unique role. At the NC-interface, we describe an elegant means by which the sidechain of a glutamine from helix 0 imitates a peptide group in order to retain hydrogen-bond continuity at the kink between helices 5 and 6 in the neighbouring subunit, thereby justifying the conservation of the helical distortion. Its absence from Cdc11, along with this structures other unusual features are critically discussed by comparison with Cdc3 and Cdc10. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=61 SRC="FIGDIR/small/537027v1_ufig1.gif" ALT="Figure 1"> View larger version (17K): org.highwire.dtl.DTLVardef@10c1eccorg.highwire.dtl.DTLVardef@ba0dacorg.highwire.dtl.DTLVardef@d7b866org.highwire.dtl.DTLVardef@8b6d7e_HPS_FORMAT_FIGEXP M_FIG C_FIG HIGHLIGHTSO_LIThe first crystal structure of a yeast septin heterodimer (Cdc3-Cdc10) provides important insights into their structural biology. C_LIO_LIIdentification of common features and differences between yeast and human septins, sheds light on the unique characteristics of yeast septin filaments. C_LIO_LIThe Cdc3G-Cdc10{Delta}1-10 crystal structure could be a crucial piece of the puzzle towards obtaining a high-resolution cryo-EM structure of the yeast septin octamer. C_LI