Robo1 Forms a Compact Dimer-of-Dimers Assembly.

Area of Science:

• Molecular and Cellular Biology
• Neuroscience
• Structural Biology

Background:

• The Roundabout (Robo) receptor and Slit ligand system mediates repulsive cues essential for neuronal development.
• Dysregulation of the Slit-Robo pathway is implicated in various cancers, presenting a therapeutic target.
• The precise mechanism of Robo receptor activation upon Slit ligand binding remains largely uncharacterized.

Purpose of the Study:

• To elucidate the structural basis of Robo1 receptor activation by the Slit2-N ligand.
• To understand how Slit binding triggers downstream signaling events in the Slit-Robo pathway.

Main Methods:

• Determined the crystal structure of Robo1 immunoglobulin domains (Ig1-4 and Ig5).
• Generated a negative stain electron microscopy reconstruction of the full Robo1 ectodomain.
• Utilized fluorescent imaging to assess Robo1 oligomerization state upon Slit2-N interaction.

Main Results:

• The Robo1 ectodomain forms compact dimers, primarily through its central immunoglobulin domains.
• These dimers can further assemble into a tetrameric structure via a 'back-to-back' interaction.
• No significant change in Robo1 oligomerization was observed upon binding to dimeric Slit2-N.

Conclusions:

• The Robo1 ectodomain adopts a dimeric and potentially tetrameric quaternary structure in the absence of ligand.
• Slit2-N binding likely induces a conformational change in Robo1, rather than altering its oligomeric state, to initiate signaling.
• This structural insight provides a foundation for understanding Slit-Robo pathway activation and developing targeted therapies.