Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 109, Issue 1, Pages 203-208Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1115109109
Keywords
signaling; structure; phototransduction; vision; transducin
Categories
Funding
- National Eye Institute [R01EY018421]
- Karl Kirchgessner Foundation
- Research to Prevent Blindness
- Lions District [20-Y1]
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Proteins segregate into discrete subcellular compartments via a variety of mechanisms, including motor protein transport, local binding, and diffusion barriers. This physical separation of cell functions serves, in part, as a mechanism for controlling compartment activity by allowing regulation of local protein concentrations. In this study we explored how soluble protein size impacts access to the confined space within the retinal photoreceptor outer segment signaling compartment and discovered a strikingly steep relationship. We find that GFP monomers, dimers, and trimers expressed transgenically in frog rods are present in the outer segment at 1.8-, 2.9-, and 6.8-fold lower abundances, relative to the cell body, than the small soluble fluorescent marker, calcein. Theoretical analysis, based on statistical-mechanical models of molecular access to polymer meshes, shows that these observations can be explained by the steric hindrance of molecules occupying the highly constrained spaces between outer segment disc membranes. This mechanism may answer a long-standing question of how the soluble regulatory protein, arrestin, is effectively excluded from the outer segments of dark-adapted rods and cones. Generally, our results suggest an alternate mode for the control of protein access to cell domains based on dynamic, size-dependent compartmental partitioning that does not require diffusion barriers, active transport, or large numbers of immobile binding sites.
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