Septin 9 has Two Polybasic Domains Critical to Septin Filament Assembly and Golgi Integrity

Abstract

Summary Septins are GTP-binding proteins involved in several membrane remodeling mechanisms. They associate with membranes, presumably using a polybasic domain (PB1) that interacts with phosphoinositides (PIs). Membrane-bound septins assemble into microscopic structures that regulate membrane shape. How septins interact with PIs and then assemble and shape membranes is poorly understood. Here, we found that septin 9 has a second polybasic domain (PB2) conserved in the human septin family. Similar to PB1, PB2 binds specifically to PIs, and both domains are critical for septin filament formation. However, septin 9 membrane association is not dependent on these PB domains, but on putative PB-adjacent amphipathic helices. The presence of PB domains guarantees protein enrichment in PI-contained membranes, which is critical for PI-enriched organelles. In particular, we found that septin 9 PB domains control the assembly and functionality of the Golgi apparatus. Our findings offer further insight into the role of septins in organelle morphology.

Bluetongue Virus Nonstructural Protein 3 Orchestrates Virus Maturation and Drives Non-Lytic Egress via Two Polybasic Motifs

“…It has been reported that PBMs act as plasma membrane export signals for some enveloped viruses, such as Murine Leukemia virus [28] and also for a non-enveloped virus [29], but has not yet been reported for an arthropod-borne virus. Interestingly, PBMs are also well known for protein signaling in the endoplasmic reticulum and Golgi apparatus [30][31][32][33]. In this study, we described a similar function as a cellular trafficking signal for the PBMs in the NS3 protein of BTV.…”

Lipid droplet–membrane contact sites – from protein binding to function

“…The presence of charged phospholipids, such as phosphoinositides, has not been well documented thus far, probably because their low concentration makes detection difficult. For example, phosphatidylinositol 3-phosphate (PI3P) mainly localizes to the ER, but has also been proposed to localize to LDs where it possibly recruits septins (Akil et al, 2016), proteins involved in the control of organelle biogenesis, shape and position (Omrane et al, 2019). Finally, charged residues on the hydrophilic face of AHs could also mediate lateral AH-AH interactions, thereby stabilizing the protein on the LD surface, as has been proposed for Plin4 (Čopičet al, 2018).…”

Production and analysis of a mammalian septin hetero-octamer complex

“…Understanding of these mechanistic differences is important as the assembly and association of these higher order components with cell membranes, actin filaments, and microtubules are fundamentally tied to septin cellular function. 24,[49][50][51][52][53][54][55] Septin hetero-hexamers and hetero-octamers both form in mammalian cells with the heterooctamers typically being the dominant species. 41,42 While the cell biology of mammalian septins is advancing rapidly (see above) and the biophysical basis of yeast septin assembly is progressing, [56][57][58][59][60][61] biochemical and biophysical studies of mammalian septins have lagged due to the lack of a mammalian hetero-octamer purification.…”