1 3 Eur Biophys J (2016) 45:861–867 DOI 10.1007/s00249-016-1175-5 BIOPHYSICS LETTER Helical model of smooth muscle myosin filament and the ribbons made of caldesmon: history revisited Apolinary Sobieszek 1,2 Received: 14 June 2016 / Revised: 23 August 2016 / Accepted: 6 September 2016 / Published online: 27 September 2016 © European Biophysical Societies’ Association 2016 this activity (by CaD and TM) was synergistic, cooperative and depended on myosin to actin ratio. Keywords Myosin filament · Helical assembly · Ribbons · Caldesmon · Tropomyosin · Regulation of actomyosin · Smooth muscle Introduction At the beginning of my scientific career, I studied the assembly of myosin filaments in extracts of vertebrate smooth muscles at low ionic strength. For the first time it was shown that these filaments possessed a regular helical arrangement of cross-bridges on their surface (Sobieszek 1972). In a parallel study, formation of semi-ordered, rib- bon-shaped structures was also observed in these extracts (Sobieszek and Small 1973). At that time there was con- troversy over the structure of myosin filaments. Our group claimed that myosin filaments are shaped like flat rib- bons, exhibiting face polarity on opposite sides of the fila- ment (Lowy and Small 1970) while others proposed that the cross-sectional profile of the filaments was round, like those in all other muscle types and that the ribbons are a fixation artifact due to hypertonicity (Somlyo et al. 1973). Being a fresh scientist in the field, I was convinced with the interpretation and we presented the results in 1972 at the Royal Society Symposia (Sobieszek and Small 1973) and the Cold Spring Harbor Symposium (Small and Sobieszek 1972; Sobieszek and Small 1972), claiming that the rib- bons in our extracts were related to the ribbon-shaped myosin filaments observed in the ultra-thin EM sections of muscle strips (Small and Squire 1972). Nevertheless, I always thought that as in all other muscle types, myosin in smooth muscle should exist in the form of helical filaments, Abstract In early studies on smooth muscle, I described a crude myosin fraction (CMF) in which self-assembly of myosin filaments was observed. For the first time, the 14-nm periodicity stemming from regular arrangement of myosin heads on the filament surface was observed (Sobieszek in J Mol Biol 70:741–744, 1972). In this frac- tion, we also observed formation of long ribbon-shaped aggregates exhibiting a 5.6-nm periodicity, characteristic of tropomyosin (TM) paracrystals (Sobieszek and Small in Phil Trans R Soc Lond B 265:203–212, 1973). We there- fore concluded that these ribbons were made of TM and they might be related to the myosin ribbons observed in electron micrographs (EM) of intact smooth muscle (Lowy and Small in Nature 227:46–51, 1970; Small and Squire in Mol Biol 67:117–149, 1972). Subsequently, Small (J Cell Sci 24:327–349, 1977) concluded that the ribbons observed in the EM sections were an artifact, but their observation in the CMF was not addressed. I have now revisited two aspects of the above studies. Firstly, based on my new multi-angle laser-scattering data and considering the length and stability of the building unit for the filament, a myosin trimer fit better to the previously proposed helical structure. Secondly, after two decades of systematic examinations of protein compositions in multiple smooth muscle extracts and isolated filaments, I concluded that the ribbons were made of caldesmon and not TM. Thirdly, actin-activated ATPase activity measurements indicated that modulation of * Apolinary Sobieszek apolinary@sobieszek.at 1 Institute for Biomedical Aging Research, Austrian Academy of Sciences, Dr. Ignaz Seipel-Platz 2, 1010 Wien, Austria 2 Nussdorfstrasse 5, 5411 Oberalm, Austria