RAC1 P29S is a spontaneously activating cancer-associated GTPase

Abstract

RAC1 is a small, Ras-related GTPase that was recently reported to harbor a recurrent UV-induced signature mutation in melanoma, resulting in substitution of P29 to serine (RAC1 P29S ), ranking this the third most frequently occurring gain-of-function mutation in melanoma. Although the Ras family GTPases are mutated in about 30% of all cancers, mutations in the Rho family GTPases have rarely been observed. In this study, we demonstrate that unlike oncogenic Ras proteins, which are primarily activated by mutations that eliminate GTPase activity, the activated melanoma RAC1 P29S protein maintains intrinsic GTP hydrolysis and is spontaneously activated by substantially increased inherent GDP/GTP nucleotide exchange. Determination and comparison of crystal structures for activated RAC1 GTPases suggest that RAC1 F28L —a known spontaneously activated RAC1 mutant—and RAC1 P29S are self-activated in distinct fashions. Moreover, the mechanism of RAC1 P29S and RAC1 F28L activation differs from the common oncogenic mutations found in Ras-like GTPases that abrogate GTP hydrolysis. The melanoma RAC1 P29S gain-of-function point mutation therefore represents a previously undescribed class of cancer-related GTPase activity.