Although cell-free extracts prepared from several types of free-living cells, including Ehrlich
tumor cells, macrophages and sea-urchin eggs, readily form
gels under low Ca2+ conditions, no such ability to induce actin-related gel has been detected in tissue-
cell extracts. Ca2+ -insensitive gelation activity was discovered, however, in several tissue-
cell extracts, including liver and brain, provided that the extracts were supplemented with skeletal muscle actin. Based on
sodium dodecylsulfate/
polyacrylamide gel electrophoretic analysis of the gel, these extracts seem to contain both a Ca2+ -insensitive gelation factor and Ca2+ -sensitive one,
actinogelin. A procedure for purification of
actinogelin from rat liver was developed, and the properties of
actinogelin thus purified were compared with those of Ehrlich
tumor cell
actinogelin. No appreciable difference was found in these two
proteins, and Ca2+ sensitivity (50% inhibition of gelation at 1 microM) was very similar. Some of the molecular characteristics are described, and the importance of the presence of
actinogelin in tissue cells is discussed.