Cells can tune the assembly of these droplets by changing the concentration of the scaffold or by modulating the solubility of the components. Typical scaffold proteins also have IDRs with high-sequence repetition composed of polar and charged amino acids or of aromatic amino acids. Scaffolds can be repeating polymers, like RNA molecules, which further increase valency through oligomerization. High-valency scaffolds tend to undergo phase separation more readily, because the entropic cost is less than for smaller proteins with fewer interaction domains ]. Interactions between these domains are stronger than the interaction between the protein and the solvent and thereby drive the phase separation process. Many scaffold proteins display high valency due to the presence of repeating interaction domains. Client proteins are mobile with respect to the scaffold and not required to induce phase separation, but are capable of modulating the properties of the separated phase.
FRAMEWORK PROTEIN SCAFFOLD DRIVERS
Scaffold proteins are the drivers of the phase separation as they tend to self-associate or have repeating units in sequence, resulting in a relatively low mobility within the separated droplet. Two principal constituents of liquid droplets are required: first scaffold proteins and second client proteins. Cells make use of phase separation to sequester specific components into one cellular location and to modulate interaction kinetics by increasing the local concentration of proteins. LLPS is a general cellular physico-chemical phenomenon involved in many biological processes and was characterized for cellular light-microscopic punctate structures such as Cajal bodies, stress granules, P bodies, and the nucleolus (recently reviewed in Ref. More recently, p62 was shown to undergo liquid–liquid-phase separation (LLPS) in vivo and in vitro ]. The complexity of the p62 interaction hub is remarkable and not easy to envision to function by simple one-to-one binding of protein interaction partners. Due to the multitude of discovered interaction partners, the p62 interaction hub is able to integrate the signals of multiple pathways such as selective autophagy ], MAP kinase ], NF-κB ], mTORC1 ], Nrf2 ], and N-end degradation ], linking p62 to many essential biological processes, such as degradation, oxidative stress, nutrient sensing, and inflammation. p62's UBA domain is critical for the recognition of poly-ubiquitin and ubiquitinated cargo ]. In addition, it contains a long intrinsically disordered region (IDR) (168–388), which provides binding sites to various interacting partners such as LC3, KEAP1, and FIP200 via short binding motifs ]. The p62 molecule is a 440 aa protein that contains three structurally folded domains: a PB1 domain (1–102), a ZZ-domain (122–167), and a UBA ubiquitin-binding domain (389–434). This so-called autophagosome is directed to the lysosome where it fuses with the lysosomal membrane and its contents are degraded. In this process, a phagophore membrane is recruited through p62's interaction with LC3 and elongated until it encloses the cargo-p62 complex in a double-membrane vesicle. Due to its early discovery, p62 can be considered the archetypical autophagy receptor that is also involved in the targeted removal of other cargo types such as bacteria, viruses, and organelles ]. The molecular link to autophagy was established by p62's involvement in the disposal of poly-ubiquitinated aggregates via the lysosome ]. Further interaction studies revealed that p62 bridges the interaction of atypical protein kinase C (PKC) with RIP1 to activate the NF-κB pathway ]. Approximately 25 years ago, p62 was identified as a novel interaction partner of the SH2 domain of tyrosine–protein kinase Lck and subsequently cloned ].
FRAMEWORK PROTEIN SCAFFOLD SERIES
P62/SQSTM1 (from here on p62) is a multidomain, multifunctional protein involved in autophagy and a series of signaling processes ]. fluorescence recovery after photobleaching.
0 kommentar(er)
