Differences in expression of a variant actin between low and high metastatic B16 melanoma.

The polypeptides of mouse B16 melanoma lines of defined metastatic potential have been analyzed by two-dimensional electrophoresis. Parent B16 melanoma and two independently isolated B16-F1 lines, which are low metastatic, exhibited a new polypeptide, Ax (pI 5.2; Mr = 43,000), comprising approximately 30% of the total actin, in addition to normal beta- and gamma-actin. The Ax is present in the Triton-insoluble fraction (cytoskeleton and nuclear matrix) as well as in the Triton-soluble fraction at a constant ratio of about 0.5 to beta- plus gamma-actin. The Ax polypeptide has been identified as a variant form of actin by immunostaining with anti-actin antibody and by a comparison of its tryptic patterns with those produced by beta- and gamma-actin polypeptides; the Ax is also identified as a component of microfilaments. On the other hand, the Ax polypeptide disappears or its expression is very low in high metastatic lines, two independently isolated B16-F10s and B16-BL6. By in vitro translation, we have identified the mRNA species that code for Ax in B16-F1, but not in B16-F10.