Abstract

Native myosin filaments were isolated in relaxing conditions from tarantula (Avicularia avicularia) striated muscle. These filaments were disassembled by suspending them into low ionic strength solutions of imidazole. The way how these filaments disassembled was compared with the reported way in which vertebrate (rat and rabbit) thick filaments frayed. Tarantula thick filaments frayed only near the tips into 4 subfilaments, whereas vertebrate thick filaments frayed into 3 subfilaments completely along the length of the filament, excluding the tip and the bare zone. Tarantula filaments frayed first into two secondary subfilamcnts: and then each of them frayed i~)to 2 more primary subfilaments. This way of fraying permits to discriminate between the 4 possible models proposed on the basis of ultrathin transverse sections for the backbone of the tarantula thick filament by Guerrero & Padron: Acta Microsc. ](2): 63-83, 1992. The selected model would have a central core (hollow or constituted by a protein different than pararnyosin or myosin) surrounded by 4 rounded features (each on constituted of 2 paramyosin molecules) and two concentric rings formed by 8 and 16 rounded features (each one constituted of 2 myosin molecules tails), totaling 48 myosin tails transversally. It is concluded that thick filaments with a central pararnyosin core like the tarantula thick filaments are more resistant to disassembly than filaments (with or without para myosin content) that do not have such central core.