PAK1, PAK1Δ15, and PAK2: similarities, differences and mutual interactions

P21-activated kinases (PAK) are key effectors of the small GTPases Rac1 and Cdc42, as well as of Src family kinases. In particular, PAK1 has several well-documented roles, both kinase-dependent and kinase-independent, in cancer-related processes, such as cell proliferation, adhesion, and migration. However, PAK1 properties and functions have not been attributed to individual PAK1 isoforms: besides the full-length kinase (PAK1-full), a splicing variant lacking the exon 15 (PAK1Δ15) is annotated in protein databases. In addition, it is not clear if PAK1 and PAK2 are functionally overlapping. Using fluorescently tagged forms of human PAK1-full, PAK1Δ15, and PAK2, we analyzed their intracellular localization and mutual interactions. Effects of PAK inhibition or depletion on cell-surface adhesion were monitored by real-time microimpedance measurement. We show that PAK1Δ15 is in many aspects similar to PAK2, rather than to PAK1-full. Both PAK1Δ15 and PAK2, but not PAK1-full, were enriched in focal adhesions, indicating that the C-terminus might be important for PAK intracellular localization. Using immunoprecipitation, we documented direct interactions among the studied PAK group I members: PAK1 and PAK2 form homodimers, but all possible heterodimers were also detected. Our results indicate that PAK1 and PAK2 have distinct roles in cell adhesion and mutually affect their function. PAK1-full is required for formation of membrane protrusions, whereas PAK2 is involved in focal adhesion assembly. We have also noted that PAK inhibition was associated with a large reduction of the cell glycolytic rate. Altogether, our data suggest a complex interplay among different PAK group I members, which have largely non-redundant functions.