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 (PAK1∆15) 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, PAK1∆15, 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 PAK1∆15 is in many aspects similar to PAK2, rather than to PAK1-full. Both PAK1∆15 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. PAK1full 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.PAK (p21-activated kinases) are a group of serine-threonine kinases originally identified as downstream effectors of p21 proteins, specifically of the Ras-related GTPases Rac1 and Cdc42 (1, 2). The initial phase of PAK research focused on their role as small GTPase effectors in the context of dynamic remodelling of the cytoskeleton and of cell adhesion structures (3). Later on, PAK were found to be involved in many cancer-related processes in different tumor types (4, 5). In parallel, the discovery of PAK1 nuclear localization (6) prompted the analysis of PAK functions in the cell nucleus.The human PAK family is divided into the group I (PAK1 to PAK3) and group II (PAK4 to PAK6). In general, these kinases regulate the cytoskeleton dynamics, intracellular signaling, and gene expression (7). The current knowledge about PAK group I is derived mostly from adherent cell models, where PAK activity usually correlates with increased cell motility. In this area, the research was mainly focused to PAK1, although some important differences between PAK1 and PAK2 have been reported (8). In addition, PAK are known regulators of a wide range of cellular processes, including the dynamics of actin structures and microtubules, cell division, apoptosis, and adhesion to the extracellular matrix.Despite considerable seque...