Two-dimensional arrays of proteins in sarcoplasmic reticulum and purified Ca2+-ATPase vesicles treated with vanadate.

The Mg2+ + Ca2+ activated ATPase of sarcoplasmic reticulum can be visualized by negative staining with uranyl acetate or Kc-phosphotungstate in the form of 40 a diameter surface particles (I), and by freeze fracture as 85 A diameter intramembranous particles (2), which are more numerous in the cytoplasmic than in the luminal fracture face. In sarcoplasmic reticulum and in reconstituted ATPase vesicles, the average density of the 40 8, surface particles is greater than that of the 85 8, intramembranous particles; this observation led to the suggestion that he 85 particles represent oligomers of several (probably four) CaZ'-ATPase molecules (3-5). While the assessment of the functional significance of ATPase-ATPase interactions is not yet completed (6) , the existence of these interactions is now generally accepted. We now report the regular formation of extensive twodimensional "crystalline" arrays of 40 A surface particles in sarcoplasmic reticulum vesicles treated with vanadate, under conditions similar to those described by Shiver et al. (7) and Hebert et al. (8) on Na",K'-ATPase. The crystalline arrays cover the entire surface of a major portion (30-50%) of the vesicles present in sarcoplasmic reticulum or purified Mg2' + Ca2+ activated ATPase preparations, and are assumed to reflect interactions between ATPase molecules, The observations provide the basis of a new approach to the study of the structure of Ca" transport ATPase within its native environment.