The variable domain from dynamin-related protein 1 promotes liquid-liquid phase separation that enhances its interaction with cardiolipin-containing membranes.

Dynamins are an essential superfamily of mechanoenzymes that remodel membranes and often contain a "variable domain" important for regulation. For the mitochondrial fission dynamin, dynamin-related protein 1, a regulatory role for the variable domain (VD) is demonstrated by gain- and loss-of-function mutations, yet the basis for this is unclear. Here, the isolated VD is shown to be intrinsically disordered and undergo a cooperative transition in the stabilizing osmolyte trimethylamine N-oxide. However, the osmolyte-induced state is not folded and surprisingly appears as a condensed state. Other co-solutes including known molecular crowder Ficoll PM 70, also induce a condensed state. Fluorescence recovery after photobleaching experiments reveal this state to be liquid-like indicating the VD undergoes a liquid-liquid phase separation under crowding conditions. These crowding conditions also enhance binding to cardiolipin, a mitochondrial lipid, which appears to promote phase separation. Since dynamin-related protein 1 is found assembled into discrete punctate structures on the mitochondrial surface, the inference from the present work is that these structures might arise from a condensed state involving the VD that may enable rapid tuning of mechanoenzyme assembly necessary for fission.

The variable domain from the mitochondrial fission mechanoenzyme Drp1 promotes liquid-liquid phase separation.

Dynamins are an essential superfamily of mechanoenzymes that remodel membranes and often contain a "variable domain" (VD) important for regulation. For the mitochondrial fission dynamin, Drp1, a regulatory role for the VD is demonstrated by mutations that can elongate, or fragment, mitochondria. How the VD encodes inhibitory and stimulatory activity is unclear. Here, isolated VD is shown to be intrinsically disordered (ID) yet undergoes a cooperative transition in the stabilizing osmolyte TMAO. However, the TMAO stabilized state is not folded and surprisingly appears as a condensed state. Other co-solutes including known molecular crowder Ficoll PM 70, also induce a condensed state. Fluorescence recovery after photobleaching experiments reveal this state to be liquid-like indicating the VD undergoes a liquid-liquid phase separation under crowding conditions. These crowding conditions also enhance binding to cardiolipin, a mitochondrial lipid, raising the possibility that phase separation may enable rapid tuning of Drp1 assembly necessary for fission.

Electrochemically Directed Assembly of Designer Coiled-Coil Telechelic Proteins.

We report the design and characterization of a de novo electrogelation protein comprising a central spider silk glue motif flanked by terminal pH-triggered coiled-coil domains. The coiled-coiled domains were designed to form intramolecular helix bundles below a sharply defined pH-trigger point (∼pH 5.3), whereas the spider silk glue protein, because of its substantial Glu content, serves both as an anionic electrophoretic transport element at neutral and elevated pH and as a disordered linker chain between the associated helix bundles at reduced pH. We show that in an electrochemical cell, a solution of these telechelic proteins migrates toward the anode where the terminal coiled-coil domains are triggered to form coiled-coil assemblies that act as transient cross-links for the e-gel state. Upon cessation of the current, the coiled-coil domains become denatured and the e-gel transforms back into a fluid solution of polypeptides in a fully reversible manner. This simplified triblock protein design mimics many of the characteristics of more complex electrogelation proteins, such as silk fibroin. As such, it provides some insight into possible general mechanisms of protein electrogelation. Moreover, this general class of electrogelation proteins has the potential for biomedical applications of electrochemically triggered gelation, such as externally switchable delivery of therapeutic cell and drugs from a responsive matrix.

Tyrosine Templating in the Self-Assembly and Crystallization of Silk Fibroin.

Native silk fibers exhibit strength and toughness that rival those of the best synthetic fibers. Despite significant research, further insight is still needed to understand the mechanisms by which silkworms are capable of spinning such tough fibers. Here we propose that π-π and π-OH group interactions of tyrosine side chains provide templating effects, such that the crystal-forming domains are in registration, thereby fostering the self-assembly of the spinning dope. Intrinsic fluorescence measurements, in conjunction with circular dichroism, showed that during self-assembly of regenerated silk solutions, the tyrosine residues were localized in a more hydrophobic local environment, suggesting preferential assembly. In situ Fourier transform infrared spectroscopy indicated that cross-linking of the tyrosine residues resulted in the development of extended ß-sheet structure. Additionally, control of cross-link density directly influenced the degree of crystallinity upon drying. Molecular dynamics simulations were performed on silk mimetic peptides in order to more thoroughly understand the role of tyrosines. The results indicated that tyrosine residues tended to transiently colocate in solution due to π-π interactions and hydrogen bonds with adjacent residues and with the peptide backbone. These more stable tyrosine interactions resulted in reduced lateral chain fluctuations and increased incidence of coordinated intrachain association, while introduction of a dityrosine bond directly promoted the formation of ß-sheet structures. In total, the experimental and modeling data support a critical role for tyrosine-tyrosine interactions as a key early feature in the self-assembly of regenerated silk protein chains and therefore are important in the robust and unusual mechanical properties ultimately achieved in the process.