STRUCTURE OF PHOSPHORYLASE-KINASE - A 3-DIMENSIONAL MODEL DERIVED FROM STAINED AND UNSTAINED ELECTRON-MICROGRAPHS

Phosphorylase kinase, the first protein kinase discovered, is a key re gulatory enzyme in glycogen metabolism. Although its biochemical prope rties are well characterized, details of its three-dimensional structu re and subunit topology are yet to be elucidated. This study describes four characteristic views of the hexadecameric holoenzyme (alpha(4) b eta(4) gamma(4) delta(4)) as observed in both negatively stained and u nstained electron micrographs. The predominant views are the widely re ported ''butterfly'' with two wing-like lobes connected by thin bridge s, and the previously described ''chalice'', composed of ''cup'' and ' 'stem'' segments. Two additional views, a ''cube'', similar to the pre viously reported ''tetrad'', and a ''cross'' or ''X'' are less common. but illustrate the overall geometry of the particle. Based on these i mages, the first three-dimensional model of the enzyme has been constr ucted, It is composed of four identical protomers that associate with D, symmetry to form the two major structural elements (the two lobes). Two protomers in a head to head arrangement make up each symmetrical lobe; to complete the holoenzyme, one lobe is inverted and placed perp endicular to the other. Thus, the overall structure has three 2-fold a xes of symmetry, and the arrangement of the four protomers approximate s a tetrahedron. Each lobe of the model corresponds to a wing of the b utterfly projection. Two projections form the chalice: in the intra-lo be orientation, one lobe forms the cup and the other forms the stem, a nd in the inter-lobe view, one-half of each lobe contributes to each s egment of the image. The cube and cross projections result from 90 deg rees rotations from the butterfly orientation. In the cube, the distal portions of each lobe are projected separately. In the cross, one lob e is crossed over and is above the other. This model both accounts for and predicts all of the observed microscopic images.