The molecular composition of square arrays.

Area of Science:

• Cell Biology
• Membrane Protein Complexes
• Biochemistry

Background:

• Square arrays are observed in plasma membranes of brain, muscle, and kidney cells.
• Previous analysis, limited to freeze fracture, indicated these arrays contain Aquaporin-4 (AQP4).
• The precise molecular composition and function of these arrays remain largely unknown.

Purpose of the Study:

• To investigate the molecular organization of AQP4-containing square arrays.
• To identify the specific AQP4 isoforms involved in forming these higher-order structures.
• To characterize the stability and composition of AQP4 square arrays.

Main Methods:

• Blue-Native PAGE immunoblots to analyze AQP4 complexes.
• 2D gel electrophoresis to examine the internal composition of arrays.
• Biochemical purification steps to assess array stability.

Main Results:

• Higher-order AQP4 complexes identified as square arrays, with AQP4 isoform M23 playing a key role.
• Square arrays appear to be aggregates of AQP4 tetramers complexed with dimers.
• Arrays contain AQP4 isoforms M23, M1, and a novel isoform Mz, and are stable during purification.

Conclusions:

• AQP4 square arrays represent stable, higher-order oligomers of AQP4.
• The M23 isoform is crucial for the formation of these arrays.
• The presence of multiple AQP4 isoforms (M23, M1, Mz) highlights the heterogeneity of these structures in vivo.