Transient Diffusive Interactions with a Protein Crowder Affect Aggregation Processes of Superoxide Dismutase 1 β‑Barrel

Aggregate formation of superoxide
dismutase 1 (SOD1) inside motor
neurons is known as a major factor in onset of amyotrophic lateral
sclerosis. The thermodynamic stability of the SOD1 β-barrel
has been shown to decrease in crowded environments such as inside
a cell, but it remains unclear how the thermodynamics of crowding-induced
protein destabilization relate to SOD1 aggregation. Here we have examined
the effects of a protein crowder, lysozyme, on fibril aggregate formation
of the SOD1 β-barrel. We found that aggregate formation of SOD1
is decelerated even in mildly crowded solutions. Intriguingly, transient
diffusive interactions with lysozyme do not significantly affect the
static structure of the SOD1 β-barrel but stabilize an alternative
excited “invisible” state. The net effect of crowding
is to favor species off the aggregation pathway, thereby explaining
the decelerated aggregation in the crowded environment. Our observations
suggest that the intracellular environment may have a similar negative
(inhibitory) effect on fibril formation of other amyloidogenic proteins
in living cells. Deciphering how crowded intracellular environments
affect aggregation and fibril formation of such disease-associated
proteins will probably become central in understanding the exact role
of aggregation in the etiology of these enigmatic diseases.