Animal evolution coincides with a novel degree of freedom in exocytic transport processes

Abstract

Exocytic transport of transmembrane receptors and secreted ligands provides the basis for cellular communication in animals. The RAB8/RAB3/RAB27 trafficking regulators function in transport processes towards the cell membrane. The small G-proteins recruit a diversity of effectors that mediate transport along microtubule and actin tracks, as well as membrane tethering and fusion. SPIRE actin nucleators organise local actin networks at exocytic vesicle membranes. By complex formation with class-5 myosins, vesicle transport track generation and motor protein activation are coordinated. Our phylogenetic analysis traced the onset of SPIRE function back to the origin of the Holozoa. We have identified SPIRE in the closest unicellular relatives of animals, the choanoflagellates, and the more distantly related ichthyosporeans. The discovery of a SPIRE-like protein encoding a KIND and tandem-WH2 domains in the amoebozoan Physarum polycephalum suggests that the SPIRE-type actin nucleation mechanism originated even earlier. Choanoflagellate SPIRE interacts with RAB8, the sole choanoflagellate representative of the metazoan RAB8/RAB3/RAB27 family. Major interactions including MYO5, FMN-subgroup formins and vesicle membranes are conserved between the choanoflagellate and mammalian SPIRE proteins and the choanoflagellate Monosiga brevicollis SPIRE protein can rescue mouse SPIRE1/2 function in melanosome transport. Genome duplications generated two mammalian SPIRE genes (SPIRE1 and SPIRE2) and allowed for the separation of SPIRE protein function in terms of tissue expression and RAB GTPase binding. SPIRE1 is highest expressed in the nervous system and interacts with RAB27 and RAB8. SPIRE2 shows high expression in the digestive tract and specifically interacts with RAB8. We propose that at the dawn of the animal kingdom a new transport mechanism came into existence, which bridges microtubule tracks, detached vesicles and the cellular actin cytoskeleton by organising actin/myosin forces directly at exocytic vesicle membranes.The new degree of freedom in transport may reflect the increased demands of the sophisticated cellular communications in animals.

The SPIRE1 actin nucleator coordinates actin/myosin functions in the regulation of mitochondrial motility

“…Phylogenetic analysis of SPIRE proteins of extant organisms traced the onset of SPIRE to the origin of the holozoa (ichthyosporea, choanoflagellates and metazoa) (Kollmar et al, 2019).…”

“…Conversely, SPIRE proteins were not detected in plants and fungi. Genome duplications during chordate evolution gave rise to up to three SPIRE genes (SPIRE1, SPIRE2, SPIRE3) and alternatively spliced exons were introduced (Kollmar et al, 2019). The mammalian genomes encode two SPIRE genes, SPIRE1 and SPIRE2.…”

“…Gene expression analysis failed to detect a transcript that has both alternatively spliced exons (Kollmar et la., 2019). All SPIRE proteins encode an N-terminal kinase non catalytic C-lobe domain (KIND), which interacts with FMN-subgroup formins, a cluster of monomeric globular actin (G-actin) binding Wiskott-Aldrich homolgy 2 (WH2) domains, a myosin 5 globular tail domain binding motif (GTBM), a RAB interaction domain (Alzahofi et al, 2020) consisting of the SPIRE-box, a modified FYVE zinc finger domain (mFYVE), and conserved C-terminal flanking sequences (Kollmar et al, 2019)(Welz and (Fig. 1B).…”

“…Research on SPIRE function in vesicle transport processes has been complicated in the past by redundant SPIRE1 and SPIRE2 functions (Pfender et al, 2011) (Alzahofi et al, 2020). The mitochondrial SPIRE1 function, however, cannot be compensated by SPIRE2 since SPIRE2 does not localise to mitochondria (Kollmar et al, 2019). A specific knockout of the mitoSPIRE1 splice variant by a deletion of exon 13 should, therefore, completely impair mitochondrial SPIRE1 function.…”

“…Vesicular SPIRE proteins function in organising actin/myosin networks to drive vesicle transport processes towards the cell periphery (Pfender et al, 2011;Schuh, 2011) (Alzahofi et al, 2020). The nature of the transport pathways is determined by the interaction of SPIRE proteins with RAB GTPases, vesicle membranes, and MYO5 motor proteins (Pylypenko et al, 2016;Tittel et al, 2015) (Kollmar et al, 2019;Alzahofi et al, 2020). The SPIRE1 exon 13encoded sequences anchor mitoSPIRE1 to the mitochondrial membrane (Manor et al, 2015)( Fig.…”