Abstract

Ultrathin transverse sections of the Flexor Metatarsis Longus muscle of the leg of tarántula Avicularia avicularia were studied by electrón microscopy in order to determine the internal structure of their thick filaments. Muscles were chemically de demembranated, fixed with glutaraldehyde in relaxing solution in the presence of 0.2 or 2% tannic acid, and then processed following standard electrón microscopical methods. We found that in the cross-bridge región the filaments present a substructure which consists of a central core of 2.8 nm diameter and 3 concentric rings (I, II and III) which are 3.3; 3.3 and 3.9 nm thick respectively. From geometric considerations and the myosin/paramyosin molar ratio we conclude that either the core is composed of a protein that is different than myosin or paramyosin, or it is hollow. The ring I could be formed by 4 features of 2 paramyosin molecules each one. Rings II and III could have 24 features form by 2 myosin tails each one. so the thick filament will have transversally a total of 48 myosin molecules and 8 paramyosin molecules. We propose a model for the axial arrangement of myosin molecules which relates the described backbone substructure to the arrangement of myosin heads on the surface of the thick filaments found from the three-dimensional reconstruction of negatively stained tarántula thick filaments.