Projection Image of Smooth Muscle α-Actinin from Two-dimensional Crystals Formed on Positively Charged Lipid Layers

Abstract

Two-dimensional crystalline arrays of chicken gizzard α-actinin have been formed on positively charged lipid layers. This is the first reported crystallization of α-actinin. The crystals have unit cell dimensions of a = 248 Å, b = 194 Å, γ = 106° and contain two α-actinin molecules. The two-sided group is P21. Projection images obtained from electron micrographs of negatively stained crystals have been calculated to a resolution of 25 Å. These images reveal a complex substructure. The molecule in projection is 340 Å in length and has 12 density peaks that probably correspond to protein domains. A pair of peaks is found at each end of the molecule, these probably correspond to the actin binding region. Eight peaks are observed in the central, rod-shaped region, these may correspond to the spectrin-like repeats predicted from the amino acid sequence. However, these eight central peaks are not arranged in four pairs but, instead, consist of three central pairs flanked at either end by a single peak, which appears larger and denser in projection than the three central pairs. The individual α-actinin molecules in projection lack 2-fold symmetry suggesting that either smooth muscle α-actinin lacks a molecular 2-fold symmetry axis or that the molecular 2-fold is not parallel with the crystallographic 2-fold axis. The ends of the molecule have different appearance in projection, suggesting that the molecule is twisted about the long axis. A hypothesis is proposed to explain the variations in molecular length and Ca2+ sensitivity between α-actinin isoforms.