Phospho‐regulation of intrinsically disordered proteins for actin assembly and endocytosis

Abstract

Actin filament assembly contributes to the endocytic pathway pleiotropically, with active roles in clathrin-dependent and clathrin-independent endocytosis as well as subsequent endosomal trafficking. Endocytosis comprises a series of dynamic events, including the initiation of membrane curvature, bud invagination, vesicle abscission and subsequent vesicular transport. The ultimate success of endocytosis requires the coordinated activities of proteins that trigger actin polymerization, recruit actin-binding proteins (ABPs) and organize endocytic proteins (EPs) that promote membrane curvature through molecular crowding or scaffolding mechanisms. A particularly interesting phenomenon is that multiple EPs and ABPs contain a substantial percentage of intrinsically disordered regions (IDRs), which can contribute to protein coacervation and phase separation. In addition, intrinsically disordered proteins (IDPs) frequently contain sites for post-translational modifications (PTMs) such as phosphorylation, and these modifications exhibit a high preference for IDR residues [Groban ES et al. (2006) PLoS Comput Biol 2, e32]. PTMs are implicated in regulating protein function by modulating the protein conformation, protein-protein interactions and the transition between order and disorder states of IDPs. The molecular mechanisms by which IDRs of ABPs and EPs fine-tune actin assembly and endocytosis remain mostly unexplored and elusive. In this review, we analyze protein sequences of budding yeast EPs and ABPs, and discuss the potential underlying mechanisms for regulating endocytosis and actin assembly through the emerging concept of IDR-mediated protein multivalency, coacervation, and phase transition, with an emphasis on the phospho-regulation of IDRs. Finally, we summarize the current understanding of how these mechanisms coordinate actin cytoskeleton assembly and membrane curvature formation during endocytosis in budding yeast.

Molecular mechanisms of force production in clathrin‐mediated endocytosis

“…Droplets form when there are multivalent interactions between components, such as between proteins with modular protein‐interaction domains, high charge densities, or intrinsically disordered regions . Recent work suggests that droplets may form at endocytic structures through interactions between intrinsically disordered prion‐like domains, which are found within several endocytic coat proteins . Depending on the composition of the droplet, its surface tension might be large and the droplet will be viscoelastic .…”

GAP-43 and BASP1 in Axon Regeneration: Implications for the Treatment of Neurodegenerative Diseases

“…Through multivalent interactions, intrinsically disordered proteins undergo liquid-liquid phase separation and form membraneless compartments that facilitate their functions (Li et al, 2012). Many proteins involved in actin assembly have intrinsically disordered regions, and phase separation mediated by these regions underlies their regulation of actin dynamics (Sun et al, 2017;Miao et al, 2018). Moreover, phosphorylation of intrinsically disordered proteins affects phase separation by modulating their charge and electrostatic interactions (Aumiller and Keating, 2016;Miao et al, 2018).…”

“…Many proteins involved in actin assembly have intrinsically disordered regions, and phase separation mediated by these regions underlies their regulation of actin dynamics (Sun et al, 2017;Miao et al, 2018). Moreover, phosphorylation of intrinsically disordered proteins affects phase separation by modulating their charge and electrostatic interactions (Aumiller and Keating, 2016;Miao et al, 2018). Given that GAP-43 and BASP1 are phosphoproteins, they may transmit signals from kinases and phosphatases to the actin cytoskeleton via phase separation.…”

Mechanisms of active regulation of biomolecular condensates

“…Whereas these long-known, floating droplet or-ganelles are large enough to be visible using simpler light microscopic techniques, in the past years liquid-liquid phase separation has been implicated in multifarious processes in which – often sub-micrometer-sized – condensates are formed at particular sites in the cell: at sites of DNA repair foci (Altmeyer et al, 2015), Polycomb-mediated chromatin silencing (Tatavosian et al, 2019), transmembrane signalling (Banjade and Rosen, 2014; Case et al, 2019), microtubule formation (So et al, 2019; Huang et al, 2018; Hernández-Vega et al, 2017; Jiang et al, 2015), actin polymerization (Weirich et al, 2017), endocytosis (Bergeron-Sandoval et al, 2017; Miao et al, 2018), transcription (Cho et al, 2018; Sabari et al, 2018; Chong et al, 2018; Boehning et al, 2018), at presynaptic active zones (Wu et al, 2019; Zeng et al, 2018), and for RNP transport (Formicola et al, 2019; Alami et al, 2014). Such localized condensates form upon a local stimulus to recruit the required set of proteins and are dissolved once the job is done.…”