Heparin binding induces conformational changes in Adeno-associated virus serotype 2

Area of Science:

• Structural biology
• Virology
• Biochemistry

Background:

• Adeno-associated virus serotype 2 (AAV2) is a widely used vector in gene therapy.
• AAV2 utilizes heparan sulfate proteoglycans (HSPGs) as its primary cellular attachment receptor.
• Understanding the AAV2-HSPG interaction is crucial for optimizing gene delivery efficiency and safety.

Purpose of the Study:

• To elucidate the structural basis of AAV2 interaction with heparin, a key component of HSPGs.
• To identify the specific binding sites and conformational changes induced upon heparin complexation.
• To provide insights into the mechanism of AAV2 cell entry.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine the structures of AAV2 alone and in complex with heparin.
• Three-dimensional image reconstruction techniques were used to achieve approximately 18Å resolution.
• Difference mapping was utilized to visualize the binding of heparin and associated structural alterations.

Main Results:

• Heparin binding was localized to two distinct regions on the AAV2 capsid: near the icosahedral twofold axes and between the threefold axes protrusions.
• The binding near the threefold axes correlates with previously identified basic residues involved in heparin interaction.
• Significant conformational changes were observed, including flattening of the threefold protrusions and widening of the fivefold channel.
• Ordered density within the capsid suggested the presence of nucleic acid, likely non-viral DNA.

Conclusions:

• AAV2 interacts with heparin at at least two distinct sites, suggesting a complex binding mechanism.
• Heparin binding induces significant conformational rearrangements in the AAV2 capsid, potentially influencing receptor-mediated entry.
• The structural insights gained can inform the design of improved AAV vectors for gene therapy applications.