Effect of single point mutations in a form of systemic amyloidosis

Abstract

Amyloid deposits of light-chain proteins are associated with the most common form of systemic amyloidosis. We have studied the effects of single point mutations on amyloid formation of these proteins using explicit solvent model molecular dynamics simulations. For this purpose, we compare the stability of the wild-type immunoglobulin light-chain protein REI in its native and amyloid forms with that of four mutants: R61N, G68D, D82I, and A84T. We argue that the experimentally observed differences in the propensity for amyloid formation result from two effects. First, the mutant dimers have a lower stability than the wild-type dimer due to increase exposure of certain hydrophobic residues. The second effect is a shift in equilibrium between monomers with amyloid-like structure and such with native structures. Hence, when developing drugs against light-chain associated systemic amyloidosis, one should look for components that either stabilize the dimer by binding to the dimer interface or reduce for the monomers the probability of the amyloid form.

Keywords: aggregation; light-chain proteins; molecular dynamics; systemic amyloidosis.

Figure 1

(A) Structure of dimer of the light-chain protein REI where yellow marks chain 1 and green does chain 2. The mutated residues are highlighted as blue = R61, red = G68, cyan = D82, and purple = A84. (B) The dimer interface of the REI light-chain protein. The second dimer interface is shown in the background, red = residues 94–101 of chain 1 and blue = residues 41–49 of chain 2. (C) Structural overlap of native and amyloid form of the REI light-chain protein.

Figure 2

Difference (ΔRMSF = RMSF_mutant − RMSF_{wild type}) between the average root-mean-square-fluctuation of a mutant and wild type. A = chain 1 and B = chain 2. Blue = R61N, yellow = G68D, maroon = D82I, and magenta = A84T.

Figure 3

Overlap of a snapshot of (A) R61N and (B) D82I mutant dimers taken in the last nanosecond with the corresponding start configuration. The residues 1–13 are part of subunit 1 and residues 45–54, 55–61, or 76–84 belong to subunit 2.

Figure 4

Relative root means square fluctuation (ΔΔRMSF) of mutants in the monomer form for native and amyloid forms calculated with respect to that of the wild type. The quantity ΔΔRMSF is defined in the main text. Blue = R61N, yellow = G68D, maroon = D82I, and magenta = A84T.

Figure 5

Structure of the light-chain protein REI, with the β-strands made out of residues: A = 4–13, B = 19–25, C = 69–75, D = 62–67, E = 53–54, F = 45–49, G = 33–38, H = 85–90, and I = 101–104. The β-strands A, B, C, and D belong to β-sheet 2 and E, F, G, H, and I belong to β-sheet 1.