Rickettsia Sca2 has evolved formin-like activity through a different molecular mechanism

Area of Science:

• Microbiology
• Cell Biology
• Structural Biology

Background:

• Surface cell antigen 2 (Sca2) is a bacterial protein essential for Rickettsia motility.
• Sca2 facilitates actin comet tail formation by nucleating and elongating actin filaments, similar to eukaryotic formins.
• The structural and mechanistic basis of Sca2's formin-like activity was previously unknown.

Purpose of the Study:

• To elucidate the unique mechanism by which Sca2 mimics eukaryotic formins for actin assembly.
• To determine the crystal structure of the N-terminal repeat domain (NRD) of Sca2.
• To understand how Sca2's distinct structural features contribute to actin nucleation and elongation.

Main Methods:

• X-ray crystallography to determine the structure of Sca2's NRD.
• Bioinformatic predictions for the structure of the C-terminal repeat domain (CRD).
• Analysis of Sca2's functional domains (NRD, CRD, proline-rich sequences, WH2 domains) in actin regulation.

Main Results:

• Sca2 is a monomeric protein with N- and C-terminal repeat domains (NRD and CRD) that interact for processive elongation.
• The NRD exhibits a novel helix-loop-helix repeat fold, forming a crescent shape.
• Proline-rich regions and WH2 domains mediate profilin-actin incorporation and actin monomer recruitment, respectively.
• Sca2's α-helical fold is atypical for Gram-negative autotransporters, which typically fold as β-solenoids.

Conclusions:

• Sca2 employs a unique mechanism to mimic formin function, involving an unusual α-helical fold.
• The NRD and CRD likely form a doughnut-shaped structure, facilitating processive actin elongation.
• Rickettsia has evolved a novel strategy to "rediscover" formin-like actin nucleation and elongation machinery.