A biochemical view on the septins, a less known component of the cytoskeleton

Abstract

The septins are a conserved family of guanine nucleotide binding proteins, often named the fourth component of the cytoskeleton. They self-assemble into non-polar filaments and further into higher ordered structures. Properly assembled septin structures are required for a wide range of indispensable intracellular processes such as cytokinesis, vesicular transport, polarity establishment and cellular adhesion. Septins belong structurally to the P-Loop NTPases. However, unlike the small GTPases like Ras, septins do not mediate signals to effectors through GTP binding and hydrolysis. The role of nucleotide binding and subsequent GTP hydrolysis by the septins is rather controversially debated. We compile here the structural features from the existing septin crystal- and cryo-EM structures regarding protofilament formation, inter-subunit interface architecture and nucleotide binding and hydrolysis. These findings are supplemented with a summary of available biochemical studies providing information regarding nucleotide binding and hydrolysis of fungal and mammalian septins.

The Evolutionary Origins and Ancestral Features of Septins

“…( A ) Topology diagram of the GTPase domain secondary structures from N to C-terminus. Conserved GTPase motifs and septin motifs are noted above by black lines (based on Grupp and Gronemeyer 2023) and the NC and G-interacting group regions are noted below by dashed lines (based on Auxier et al 2019). The typical position of the R-finger (when present) is indicated by the pink star.…”

An in silico approach to determine inter‐subunit affinities in human septin complexes

“…What is the difference in the two complexes that might explain this phenomenon? SEPT2 and SEPT7 are both GTPase active whereas SEPT6 is not (reviewed in Grupp and Gronemeyer (2022)). Consequently, our finding implies that mainly the GDP from the two GTPase active subunits contributes to the binding affinity.…”

“…This is in perfect agreement with the above described Arg(βb) flipping which occurs only in the GTPase active subunits upon nucleotide removal. We used The role of nucleotide hydrolysis in septins is still poorly understood and subject of debate (Grupp & Gronemeyer, 2022). GTP hydrolysis was shown by biochemical assays for members of the Momany 1a and 2b groups, but hydrolysis products are not released to the surrounding medium by native hexameric or octameric complexes (Fischer, Frank, Rösler, Johnsson, & Gronemeyer, 2022).…”

Anin silicoapproach to determine inter-subunit affinities in human septin complexes

“…What is the difference in the two complexes that might explain this phenomenon? SEPT2 and SEPT7 are both GTPase active whereas SEPT6 is not (reviewed in (Grupp and Gronemeyer 2022)). Consequently, our finding implicates that only the GDP from the two GTPase active subunits contributes to the binding affinity.…”

“…The role of nucleotide hydrolysis in septins is still poorly understood and subject of debate (Grupp and Gronemeyer 2022). GTP hydrolysis was shown by biochemical assays for members of the Momany 1a and 2b groups, but hydrolysis products are not released to the surrounding medium by native hexameric or octameric complexes (Fischer et al 2022).…”