Septins throughout phylogeny are predicted to have a transmembrane domain, which inCaenorhabditis elegansis functionally important

Abstract

SUMMARYSeptins, a conserved family of filament-forming proteins, contribute to eukaryotic cell division, polarity, and membrane trafficking. Septins are thought to act in these processes by scaffolding other proteins to the plasma membrane. The mechanisms by which septins associate with the plasma membrane are not well understood but can involve two polybasic domains and/or an amphipathic helix. We discovered that the genomes of organisms throughout phylogeny, but not most commonly used model organisms, encode one or more septins predicted to have transmembrane domains. The nematodeCaenorhabditis elegans, which was thought to express only two septin proteins, UNC-59 and UNC-61, translates multiple isoforms of UNC-61, and one isoform, UNC-61a, is predicted to contain a transmembrane domain. UNC-61a localizes specifically to the apical membrane of theC. elegansvulva and is important for maintaining vulval morphology. UNC-61a partially compensates for the loss of the other two UNC-61 isoforms, UNC-61b and UNC-61c. The UNC-61a transmembrane domain is sufficient to localize a fluorophore to membranes in mammalian cells, and its deletion from UNC-61a recapitulates the phenotypes ofunc-61anull animals. The localization and loss-of-function phenotypes of UNC-61a and its transmembrane domain suggest roles in cell polarity and secretion and help explain the cellular and ssue biological underpinnings ofC. elegansseptin null alleles’ enigmatically hypomorphic phenotypes. Together, our findings reveal a novel mechanism of septin-membrane association with profound implications for the dynamics and regulation of this association.

The Evolutionary Origins and Ancestral Features of Septins

“…This distribution of TM domains in our dataset seems to suggest that they emerged early in the non-opisthokont branch after its split with opisthokonts and were subsequently lost in many species in Group 6B and 8. [See, however, Discussion for another possibility given a recent report by (Perry et al, 2023). ] It is interesting to note that there is little overlap between the distributions of CC and TM domains in Group 6 septins (Fig.…”

“…We initially interpreted this as evidence that the TM domain emerged after the opisthokont/non-opisthokont split and was subsequently lost in some lineages. However, a recent study by Perry et al (2023) reported the presence of TM domains in a transcript isoform of C. elegans UNC-61 (Group 1) as well as many other opisthokont proteins currently annotated as septins on the Uniprot database (but were not included in our list of 254 septins). Interestingly, many of these TM domains are found in the NTE, as seen in C. anguillulae A0A1Y2I4M7 (Fig.…”

The evolutionary origins and ancestral features of septins

“…This distribution of TM domains in our dataset seems to suggest that they emerged early in the non-opisthokont branch after its split with opisthokonts and were subsequently lost in many species in Group 6B and 8. [See, however, Discussion for another possibility given a recent report by ( Perry et al, 2023 ).] It is interesting to note that there is little overlap between the distributions of CC and TM domains in Group 6 septins ( Figure 7D ), perhaps suggesting that the evolution of septin-septin interactions through CC domains necessitated a concomitant loss of TM that would otherwise restrict the accessibility of CTE.…”

“…We initially interpreted this as evidence that the TM domain emerged after the opisthokont/non-opisthokont split and was subsequently lost in some lineages. However, a recent study by Perry et al (2023) reported the presence of TM domains in a transcript isoform of C. elegans UNC-61 (Group 1) as well as many other opisthokont proteins currently annotated as septins on the Uniprot database (but were not included in our list of 254 septins). Interestingly, many of these TM domains are found in the NTE, as seen in C. anguillulae A0A1Y2I4M7 ( Figure 7E ).…”

DataSheet1.zip

“…This distribution of TM domains in our dataset seems to suggest that they emerged early in the non-opisthokont branch after its split with opisthokonts and were subsequently lost in many species in Group 6B and 8. [See, however, Discussion for another possibility given a recent report by (Perry et al, 2023).] It is interesting to note that there is little overlap between the distributions of CC and TM domains in Group 6 septins (Figure 7D), perhaps suggesting that the evolution of septinseptin interactions through CC domains necessitated a concomitant loss of TM that would otherwise restrict the accessibility of CTE.…”