The enzyme transfers a methyl group to the 10-position of a ¦¤-olefinic acyl chain in phosphatidylglycerol or phosphatidylinositol or, more slowly, phosphatidylethanolamine; subsequent proton transfer produces a 10-methylene group (cf. EC 2.1.1.79 cyclopropane-fatty-acyl-phospholipid synthase).