On the Structural and Biochemical Properties of the Human Septins: SEPT3

Abstract

Septins constitute a family of conserved guanine nucleotide binding proteins that are found in different organisms. The human septins comprise 13 genes which encode proteins involved in important cellular processes, such as cytokinesis, exocytosis, membrane compartmentalization. They can be divided into 4 groups based on sequence similarity. Septins from different groups have showed to interact with each other assembling into heteromeric complexes that can polymerize to form filaments. Currently, only one of these possible heterocomplexes had its structure solved, but it did not include any member of the SEPT3 group. Recent publications suggest SEPT9, a member of the SEPT3 group, can occupy the terminal position in the assembly of octameric complexes. Here, we present the first determination of the crystallographic structure and biochemical properties of the SEPT3. The analysis of this structure shows unique features that can be useful in understanding the assembling of septin octamer units and the polymerization control of the septin filaments. A truncated version of SEPT3 lacking the N-terminal domain (SEPT3-GC) was found to be a monomer in solution, and the dimerization was dependent on salt concentration and the presence of GTPS. Additional site-directed mutagenesis experiments proved that the SEPT3-GC monomeric state is related to a tyrosine residue which is present in all human septins, except in the SEPT3 group. The SEPT3-GC affinity by GTPS was Mg2+ dependent and this septin was also active and able to hydrolyze GTP in vitro. The crystal structure of SEPT3-GC forms foreshortened filaments which employ the same NC and G interfaces observed in the hexameric complex of human septins 2, 6 and 7, reinforcing the notion of ‘promiscuous' interactions described previously.