of GFGNFGTS from the porin nup98, confirming
LARKS architecture (Fig. 1F) and providing evidence that LARKS are present in a different type
of membraneless organelle (27).
Analyzing the nonredundant human proteome
of 20,120 sequences from UniProt, we found 5867
proteins with LCDs. Of these, 2500 proteins contain at least one LARKS and 1725 proteins contain two or more LARKS and thus are able to
Fig. 1. Structures of LARKS compared to a steric zipper. (A) Steric zipper. (B) to
(F) Structures of LARKS. All structures are composed of two mating b sheets, one purple
and the other yellow. The left-hand column shows the trace of the backbones of
mating sheets to highlight kinks in the backbones of LARKS and the pleating of the classical
b sheets in steric zippers. The second column shows the atomic structures of mating
sheets viewed down the fibril axes. The third column shows cartoons of the mating
b sheets viewed nearly perpendicular to the fibril axes. Each interface is characterized by
the shape complementarity score (Sc = 1.0 for perfect complementarity) and buried
solvent-accessible surface area (Ab) in Å2 between the mated sheets. Carbon atoms are
colored purple or yellow, nitrogen is blue, and oxygen is red. Five layers of b sheets are shown
of the hundreds of thousands in the crystals. The kinked structures of LARKS are rare
among mating b sheets; dozens of other paired b sheets form steric zippers (35).
Fig. 2. Synthetic LARKS construct forms a
labile hydrogel. A synthetic LARKS construct
with the sequence SYSGYSGDT
SYSSYGQSNGPSTGGYG (underlined sequences
correspond to LARKS) forms a labile hydrogel
when dissolved in water at 50 mg/ml and left
overnight at 4°C. The hydrogel melts upon
heating the sample to 60°C for 2 hours. A
bubble (blue arrow) was introduced to the
sample to show the difference between the
liquid state (bubble rises) and hydrogel state
(bubble does not rise). Electron microscopy
confirms that fibrils were indeed melted.
The hydrogel-forming property of this
triple-LARKS sequence suggests that it is the
multiple LARKS found in many LCDs that endow
their unusual property of forming hydrogels.
Scale bars, 200 nm. Single-letter abbreviations
for the amino acid residues are as follows:
A, Ala; C, Cys; D, Asp; E, Glu; F, Phe; G, Gly;
H, His; I, Ile; K, Lys; L, Leu; M, Met; N, Asn; P,
Pro; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val;
W, Trp; and Y, Tyr.