Identification of SP-R210 as MyoXVIIIA and coimmunoprecipitation with MyoIIA. A, control, MyoXVIIIAβ/SP-R210S (A1), and MyoIIA-expressing COS-1 (A2) cells were obtained as described under “Experimental Procedures” and were probed with MyoXVIIIAct, MyoIIA, or SP-R210 antibodies on Western blots. Control cells were transfected with plasmids having either MyoXVIIIAβ/SP-R210S or MyoIIA cDNA cloned in the antisense orientation. Each lane was loaded with Laemmli lysates from 70,000 cells. B, cell lysates from mAM cells were incubated with MyoXVIIIAct antibodies, and immunoprecipitated (i.p.) proteins were analyzed on Western blots (w.b.) using SP-R210, MyoXVIIIAct, or MyoIIA antibodies as indicated. Proteins were separated on 7% SDS-polyacrylamide gels; the gels were allowed to run off until the 37-kDa marker reached the bottom of the gel to allow separation of long and short isoforms of MyoXVIIIA.