Ordered transmembrane and extracellular structure in squid photoreceptor microvilli

Area of Science:

• Structural biology
• Photoreceptor research
• Biophysics

Background:

• Invertebrate retinas possess hexagonal microvilli arrays forming rhabdome photoreceptors.
• Squid retinas exhibit exceptionally large and crystalline rhabdomes, ideal for structural analysis.

Purpose of the Study:

• To model the electron density distribution in microvillar membranes of squid retinas.
• To investigate the structural basis for the high sensitivity of these photoreceptors to polarized light.

Main Methods:

• Low angle X-ray diffraction was employed to analyze membrane structure.
• Correlation averaging of electron microscope images aided in model derivation.
• Electron microscopy revealed detailed membrane junctional structures.

Main Results:

• A model electron density map showed high protein content in microvillar membranes.
• Dense meshworks of membrane junctions, including Y-shaped contacts, were identified.
• These junctions form a 3D array, partitioning membranes into deformable and rigid domains, maintaining long-range order.

Conclusions:

• The observed structural order in squid rhabdomes is crucial for rhodopsin alignment and high sensitivity to polarized light.
• Rhodopsin comprises approximately half the microvillar protein, with other proteins forming novel junctional structures.