Tubular organization of crystalline argininosuccinase

Abstract

1. 1.|Argininosuccinase ( l-argininosuccinate arginine-lyase, EC 4.3.2.1) from bovine liver crystallizes in the form of fine needles. When viewed by electron microscopy these are seen to be paracrystals, each needle consisting of a spindle-shaped bundle of loosely packed, hollow tubules. The tubules have a uniform width of 210 Å and a wall thickness of 65 Å. A set of helices (Set 1), placed almost perpendicularly to the long axis of the tubules and having a periodicity of 50 Å is clearly resolved in whole mount preparations. 2. 2.|Optical diffraction analysis indicates that the repeat units in the tubular walls are arranged in the form of a single basic helix that probably has about 3.5 repeat units per turn (see next paper 1). Independent physical data have shown that the enzymatically active form is a dimer consisting of two similar subunits. Calculations based on the diffraction analysis indicate that the repeat units of the tubule can be identified with the active enzyme. 3. 3.|Electron microscope images of samples collected during early stages in the formation of the paracrystals suggest a sequence in the self assembly of the tubules. Tubule formation begins with the linear aggregation of enzyme molecules into short, irregular ribbon-like sheets, one molecular layer in thickness; these elongate and close over to form tubules which become grouped into loosely packed bundles.