Rim1 and rabphilin-3 bind Rab3-GTP by composite determinants partially related through N-terminal alpha -helix motifs

Area of Science:

• Neuroscience
• Molecular Biology
• Cell Biology

Background:

• Rim1 is a presynaptic active zone protein essential for neurotransmitter exocytosis.
• Rab3, a small GTPase, plays a critical role in neurotransmitter vesicle trafficking.
• Understanding the molecular basis of Rim1-Rab3 interaction is key to synaptic function.

Purpose of the Study:

• To identify the specific molecular determinants within Rim1 responsible for binding Rab3.
• To characterize the binding affinity and specificity of the Rim1-Rab3 interaction.
• To compare the Rab3-binding properties of Rim1 with related proteins like rabphilin-3 and aczonin.

Main Methods:

• Surface plasmon resonance (SPR) using recombinant proteins.
• Bacterial expression and purification of Rab3 and Rim1 proteins.
• Pull-down assays with native brain lysates under different detergent conditions.

Main Results:

• A short N-terminal alpha-helix (amino acids 19-55) of Rim1 specifically binds GTP-saturated Rab3 isoforms A, C, and D with high affinity (K(d) = 1-2 µM).
• A point mutation (R33G) within this helix abolished Rab3 binding, confirming its critical role.
• Rim1's zinc finger domain binds Munc13, and downstream sequences bind syntaxin and Rab3 (GTP-inhibited) in brain lysates.

Conclusions:

• The N-terminal alpha-helix of Rim1 is the primary Rab3-binding site, essential for regulating vesicle dynamics.
• Rim1 exhibits distinct binding properties compared to rabphilin-3 and aczonin, highlighting unique functional roles.
• These findings provide molecular insights into the organization and function of the presynaptic active zone.